Supplementary Note Name:_____________ Class:_______ ( )

Ch 23 Infectious diseases and disease

prevention
n Infectious diseases can be transmitted from one person to another.

n Three main factors contributing to the occurrence of infectious diseases:

1) Pathogen: infective agent causing the disease
2) Host: an organism at risk of infection by the pathogen
3) Environment: conditions that allow the transmission of the pathogen to host
à Infectious diseases can be prevented if any one of the factors is under control.

23.1 Pathogens of infectious diseases

n Pathogens [everywhere]: found in soil, air, contaminated water, infected people and animals
à Enter the human body through the eyes, open wounds or other openings
à Can also overcome the body defence and multiply in the host
à Causing disease when the pathogens cause damage to the host cells or tissues
à Example: viruses, microorganisms (bacteria, protists, fungi)

A Viruses
n Characteristics: small, most of them visible under electron microscope, non-cellular particles

Nucleic acids

Protein coats

Envelopes *

surface protein
n recognizes and attaches to
specific cell types of their
‡ General structure of a virus hosts ‡ Influenza virus (×210 000)

n Viruses cannot multiply on their own. Instead, viruses introduce their nucleic acid into
living host cells. The viral nucleic acid takes over the cellular mechanism of the host cells to
produce viruses. The newly formed viruses are then released from the host cells to infect more
cells.
à disrupting cellular mechanism or damaging cell membrane of host cells à disease
n Examples of viral diseases: influenza, dengue fever, measles, hepatitis B and acquired immune
deficiency syndrome (AIDS)

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Extended learning:
n Newly formed viruses are released from the host cell by lysis (causing the host cell to burst) or
budding off from the cell membrane.
n Coronavirus diseases 2019 (COVID-19): caused by severe acute respiratory syndrome
coronavirus 2 (SARS-CoV2). The virus is mainly spread through droplets or contact.
n Although not all viruses infect humans, new viral diseases can emerge when existing viruses
from other animals undergo mutations and spread to human.
1) Middle East Respiratory Syndrome (MERS): first appeared in Saudi Arabia in 2012. Since
then, there have been over 2000 reported cases in 27 countries and about 25% of patients died.
Studies have shown that camels are major source of the virus.
2) Two cases of human infection of rat hepatitis E virus occurred in 2018. The spread of this
virus from rats to humans had never been reported before.

B Bacteria
n Characteristics: larger than viruses, most of them visible under high-power of light microscope,
unicellular prokaryotes (no true nucleus and membrane-bound organelles), reproduce quickly
under favourable conditions (binary fission)

capsule (protective coat) *

cytoplasm cell wall
cell membrane
circular DNA
ribosome

flagellum / flagella *
(movement)
‡ General structure of a bacterium

n Some bacteria are pathogenic and mainly cause diseases by releasing toxins to disturb the
normal functioning of human cells (Example: bacteria that cause cholera release a toxin to
induce fluid loss from intestinal cellsàdiarrhea and dehydration in patients).

n Extended learning:
1) The toxin from the cholera bacteria chemically alters the channel proteins in the cell
membrane of the epithelial cells of the small intestine. à Lots of chloride ions in the epithelial
cells are released into the lumen of small intestine. à Lower the water potential in the lumen
à Water moves out of the blood, across the epithelial cells and into the lumen by osmosis

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2) “Flesh-eating” bacteria: present naturally in warm seawater, produce powerful toxins à

destroy skin, muscles and fats à trigger immune system to attack own tissues. A limb can be
destroyed in hours and removal of the infected limb by surgery may be needed to stop rapid
destruction of tissues. Higher risk: people with open wound exposed to seawater, people
consuming raw or uncooked seafood contaminated with the bacteria

C Protists
n Characteristics: larger than bacteria, high-power of light microscope, unicellular eukaryotes
(with true nucleus and membrane-bound organelles, but no cell wall)
n They are called protozoans. Some protozoans can live inside the human body as parasites à
cause diseases by direct destruction of human cells or releasing toxins
n Some protozoans can live inside the human body as parasites. They cause diseases by direct
destruction of human cells or releasing toxins.
n Example: Plasmodium is a protozoan that causes malaria
Ø The parasites multiply in the human liver cells and then in red blood cells. When the
infected red blood cells burst, newly formed parasites and toxic substances are released,
causing shivering, fever and sweating in patients.

D Fungi
n Characteristics: larger, unicellular (yeast) and multicellular (moulds) eukaryotes (with true
nucleus and membrane-bound organelles, cell wall composed of complex polysaccharides
including chitin, glycoprotein, glucans)

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n Fungi produce spores for reproduction which are usually air-borne and can resist adverse
conditions (e.g. high temperatures). Spores will germinate into new fungi when conditions
become favourable.
n Some fungi secrete enzymes to digest human tissues and then absorb the digested products for
their growth.
n Example of disease caused by fungi: Athlete’s foot (moulds), living on the warm and moist skin
between toes. The actions of enzymes help their thread-like hyphae penetrate the skin à causing
red, cracked, and itchy.

E Other pathogens
n Some non-microscopic pathogens (not microorganisms), eg. parasitic worms (liver flukes
living in the bile duct à causing inflammation and cancer in bile duct

Summary of different types of pathogens

Types of Main characteristics Way of causing diseases Example of diseases
pathogens
Viruses small, non-cellular By disrupting the cellular mechanism Influenza, Dengue
particles, multiply by and damaging the cell membranes of feverm Measles,
taking over cellular human cells Hepatitis B, AIDs,
mechanisms COVID-19
Bacteria unicellular By releasing toxins which disturb the Cholera
prokaryotes normal functioning of human cells
Protists unicellular By direct destruction of human cells or Malaria
eukaryotes releasing toxins
Fungi unicellular and By secreting enzymes to digest human Athlete’s foot
multicellular tissues
eukaryotes

Identifying pathogen (for reference)- Koch's four postulates:

1. Microorganism must be present or associated with every case of a disease but absent from a healthy
organism. [However: some are beneficial microorganisms, natural parasite of the animals, some
person may be carriers of the pathogen or resistant to the pathogens]
2. The suspected microorganism must be isolated and grown in pure culture.
3. Inoculation of a healthy host of the same isolated microorganism must produce exactly the same

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Supplementary Note

disease. [However: human should not be used due to ethical concern]

4. The same microorganism must be re-isolated from the new diseased host.
à identification of pathogens of an infectious diseases using Koch’s postulates enables doctors to
treat diseases using antibiotics and support the manufacture of antibodies

23.2 Transmission and prevention of infectious diseases

n Pathogen can be transmitted from one host to another in one or more ways
à can be reduced by taking appropriate preventive measures

A By droplets or air
Ø Droplets are expelled from our airways into the air when walking, coughing or sneezing
Ø Single cough: about 3000 droplets; a sneeze: around 40000 droplets may be expelled

n Pathogens are transmitted when…

Ø droplets that contain pathogens fall on the mucous membranes of the eyes, nose or mouth
of another person;
Ø a person rubs their nose with hands contaminated with droplets;
Ø tiny droplets, dust particles and fungal spores (light weight) which suspend in the air
or travel in air currents over long distances are inhaled into the body.

B By water or food
n Infectious diseases can occur when water or food contaminated with pathogens is ingested.
n Water and food are contaminated when:
Ø Sewage or excreta from infected persons enter the water supply system;
Ø Meat, milk and eggs are obtained from infected animals;
Ø Crops are fertilized using excreta from infected persons;
Ø Food is not handled properly.
Ø Food with a higher risk causing food-poisoning, eg. raw food (oysters, sashimi, green
salads) or food made with raw egg (tiramisu)

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C By vectors
n A vector is an animal that carries pathogens to a new host.
Ø Rodents: can transmit diseases (e.g. typhus) through bites, direct contacts with rodents or
their excreta, and ingestion of food contaminated with their excreta.
Ø Cockroaches: can transmit diseases (e.g. cholera) when they pick up pathogens from the
faeces of an infected person and transfer them to food they touch.
Ø Mosquitoes: can transmit diseases (e.g. malaria, dengue fever, Japanese encephalitis, Zika
virus infection) when they take in pathogens as they suck blood from an infected person
and then bite other people.

Extended learning:
Ø Plague was responsible for widespread pandemics with high mortality rate. It is caused
by bacteria usually found in small mammals and their fleasà transmitted by the bite of
infected fleas. Major events: Black Death in Europe of 14th century leading to more than
50 million death, outbreak of plague in HK in 1894 leading to 2000 death.
Ø Most female adult mosquitoes need additional nutrients from mammalian blood to
produce eggs. Male adult mosquitoes feed on plant juices only.
Ø Mosquitoes mainly lay eggs in stagnant water. The larvae and pupae hang just below the
water surface and obtain air through breathing tubes.

D By body fluids
n Examples of body fluids include blood, semen and vaginal secretion.
Ø Blood from an infected person may enter the bloodstream of another person through
wounds, sharing of injection needles, transfusion or during childbirth.
Ø Semen and vaginal secretion are exchanged during sexual intercourse.
n If these body fluids contain pathogens, the transmission of diseases is possible.
n Examples: Hepatitis B and AIDS (sexually transmitted disease – virus could not live outside the
human body, ordinary social contact eg. shaking hands will not transmit the virus)

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Supplementary Note

E By direct contact
n Direct contact transmission involves physical contact between an infected person and a
non-infected person. This occurs through contact with skin, wounds or mucous membranes,
kissing and sexual intercourse.
n Examples: Althelet’s foot and genital herpes (STDs – can be transmitted with or without
symptoms)

Ways of transmission of infectious diseases and preventive measures

Way of
Example of disease Preventive measure
transmission

By droplets or air n Influenza n Cover our mouth with tissue paper when
n SARS coughing or sneezing.
n Measles n Wear a face mask if having any respiratory
n Tuberculosis (TB) symptoms, such as running nose.
n Wash our hands with liquid soap and water,
especially after coughing and sneezing.
n Maintain good ventilation.
n Avoid going to crowded places when
air-borne diseases are prevalent.

By water or food n Cholera n Dispose of sewage and excreta properly.

n Gastroenteritis n Ensure drinking water is clean.
n Hepatitis A n Handle food properly (cook thoroughly,
separate raw and cooked food in fridge).
n Maintain good personal and environmental
hygiene (clean utensils).

By vectors n Malaria 1) Remove hiding or breeding places:

(e.g. mosquitoes) n Dengue fever n Clear stagnant water (breeding place).
n Japanese n Discard the waste regularly to remove hiding
encephalitis places for rodents and cockroaches.
n Zika virus infection 2) To kill the vectors:
n Use pesticides or larvicidal oil to kill the
adult mosquitoes or their larvae.
n Use poisonous bait to kill rodents and
cockroaches.

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Supplementary Note

3) To avoid contact with the vectors:

n Wear gloves and use tongs when handling
dead rodents.
n Avoid mosquito bites by using insect
repellent, mosquito nets, wearing
light-coloured and long-sleeved tops and
trousers for outdoor activities

By body fluids n Hepatitis B n Wear gloves when handling wounds.

(e.g. blood, n AIDS n Cover any wound with a dressing.
semen, vaginal n Never share injection needles.
secretion) n Screen blood used for transfusion.
n Stay with one sexual partner.
n Use a condom correctly.

By direct contact n Athlete’s foot n Minimize physical contact with infected

(e.g. skin) n Genital herpes persons.
n Maintain good personal hygiene.

Extended learning:
n Aedes albopictus is the vector of dengue fever. The Food and Environmental Hygiene
Department places ovitraps at different locations in Hong Kong. Ovitrap Indices are obtained
using the percentages of the ovitraps with mosquito eggs or larvae. The indices, classified
into levels 1 to 4, reflect the extensiveness of mosquito distribution in particular area and the
overall local situation à corresponding preventive and control measures can be carried out

23.3 Treatment of infectious diseases

n Involves the use of drugs
à antibiotics and sulpha drugs

A Antibiotics
n Antibiotics are used for treating bacterial diseases.
n They can kill bacteria or inhibit their growth. (NOT virus)
n In nature, antibiotics are produced by certain bacteria and fungi to reduce the growth of
competing microorganisms. This allows more resources for their own survival.

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1 Discovery of antibiotics
n In 1928, Sir Alexander Fleming discovered that the fungus Penicillium produced an
antibacterial substance. He named this substance penicillin.
n Story: Fleming grew bacteria on agar plate and one of the plates was contaminated by fungus.
He noticed that there was no bacterial growth around the fungus. He thought the fungus had
released a substance that stopped the bacteria from growing. Fleming grew the fungus along
with different bacteria and found that some bacteria did not grow. He repeated using other fungi
but no similar antibacterial effect was found.
n About a decade later, Ernst Boris Chain and Sir Howard Florey isolated penicillin and tested its
safety for use. Since then, penicillin and other antibiotics, either natural and synthetic, have
become available as drug.
n Fleming, Chain and Florey were awarded the Nobel Prize in Physiology or Medicine in 1945
for the contribution to the discovery and development of penicillin. [NOS: scientists build on the
work of other scientists]

2 Actions of antibiotics
Action on bacteria Consequence

Inhibit the formation of cell walls The bacteria lyse (cell wall cannot withstand the
pressure) when water enters the cells by osmosis.

Damage cell membranes Cell contents leak and the bacteria die.

Inhibit the synthesis of nucleic acids The bacteria cannot carry out cell division.

Inhibit protein synthesis The bacteria cannot function and grow.

**Since the structures of prokaryotic cells and eukaryotic cells are different, antibiotics can kill
bacteria or inhibit their growth without causing harm to human cells. (e.g. human cells do not have
cell walls)

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3 Effectiveness of antibiotics against bacteria

1) Broad spectrum antibiotics act on a wide variety of bacteria
2) Narrow spectrum antibiotics act on only a limited variety of bacteria

Comparing the effects of antibiotic discs on bacterial growth

Ø Assessed by the diameter of the clear zone formed around the antibiotic disc
Ø The antibiotic diffuses outwards from the paper disc into the agar à creating the gradient of
antibiotic concentration (the farther away from the disc, the lower the concentration of
antibiotic in the agar)
Ø Larger clear zone indicates that the antibiotic is effective against the bacteria even at a
lower concentration.

Ø Controlled variables: concentrations of antibiotics, incubation time, incubation temperature,

kind of bacteria, amount of bacteria
Ø Disc with distilled water: any formation of clear zones / inhibition of bacterial growth is due
to the actions of antibiotics [should have no clear zone, no effect on bacterial growth]
Ø Antibiotic C is most effective against the bacteria in the test (diameter is longest)
Disc A B C D E
Diameter of clear zone (mm) 11 10 15 12 0

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4 Indiscriminate use of antibiotics

Since the discovery of antibiotics were used in the 1940s, they were widely used as they were
effective in treating certain infections and infectious diseases. However, the indiscriminate use
of antibiotics has led to adverse consequences.

i) Examples of indiscriminate use

n Doctors prescribe antibiotics unnecessarily when their patients have viral infections.
n Patients do not complete their course of antibiotics when they start to feel better.
n Livestock are fed with antibiotics to prevent the spread of disease and promote their growth.

ii) Adverse consequences of indiscriminate use ***

n The indiscriminate use of antibiotics increases the exposure of bacteria to antibiotics. This
exerts a strong selection pressure, speeding up the development of antibiotic resistance in
large populations of bacteria. [development of antibiotic resistance in bacteria is an
evolutionary process]

n How indiscriminate use of antibiotics speeds up the development of antibiotic resistance in

bacteria:

non-resistant
bacterium

resistant
bacterium

1 In a population of 2 When an antibiotic is used, 3 When antibiotics are

bacteria, most bacteria are the resistant bacteria have a indiscriminately used, the
non-resistant to antibiotic higher chance to survive proportion of resistant
initially, while some are and reproduce. The bacteria in the population
antibiotic-resistant. non-resistant bacteria will be increases in subsequent
killed. generations.

n Antibiotic-resistant bacteria can spread from animals and crops to people, and from person to
person.

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Supplementary Note

In 2016, the Consumer council tested

chicken products sold in the market
to study the presence of antibiotic
resistant bacteria (ESBL). The
results showed that over 60% if the
products contained this species of
bacteria. Prevention: cook the meat
thoroughly, handle raw meat
properly.

n As antibiotic resistance develops rapidly in bacteria, many antibiotics are losing their
effectiveness. When bacteria become resistant to most commonly used antibiotics, they are
called multi-drug-resistant organisms (MDROs) or ‘superbugs’. They pose a great threat to
global health and the economy.

n Problems:
1) Previously treatable diseases may become untreatable à higher mortality rate
2) Antibiotics that are more expensive and cause more side effects have to be used à
treatment of patients becomes more difficult
3) Many resources are allocated for the development of new antibiotics
4) Patients may have longer hospital stays
n The indiscriminate use of antibiotics also kills beneficial bacteria in our body. In the absence of
beneficial bacteria, resistant bacteria may overgrow and cause diseases.
n Narrow spectrum antibiotics is more recommended to treat disease with known bacteria as
this prevents non-pathogenic bacteria in the body from being affected by the action of
antibiotic.

iii) Actions to ease the adverse consequences

n Reduce the exposure of bacteria to antibiotics.
Ø Doctors only prescribe antibiotics when they are needed.
Ø Livestock farmers minimize antibiotics use in rearing livestock.
n Ensure all pathogenic bacteria are killed when antibiotics are used.
Ø Patients complete the course of antibiotics even symptoms have disappeared.
Ø Healthcare workers remind patients to take the right dose of antibiotics at right time.

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Supplementary Note

n Prevent the spread of antibiotic-resistant bacteria.

Ø Patients maintain good hand hygiene.
Ø Healthcare workers perform goof infection control measures.

Extended learning:
Ø Staphylococcus aureus is the most common causes of wound infections
Ø In 1960s, these infections could be treated successfully using antibiotic methicillin.
Ø As repeated use for many years, resistance to this and other commonly used antibiotics
began to develop rapidly on S.aureus found in hospitals.
Ø To make the situation worse, this methicillin-resistant S.aureus (MRSA) has spread
from hospitals into the community à The bacterium can spread from person to person
by direct contact and through contaminated objects.
Ø It often causes skin infections, also can cause pneumonia and heart valve infections.

B Sulpha drugs
n Sulpha drugs are chemicals derived from a sulphur-containing red dye. They have been used in
the treatment of bacterial infections.
n Sulpha drugs are inhibitors of certain enzymes in bacteria, stopping or slowing down the
metabolic reactions. They can inhibit the growth of the bacteria.

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23.4 Disease prevention

n Disease prevention requires joint efforts from both individuals and the community from actual
or potential health threats.

A At individual level
Action Example Contribution

Adopt healthy n Eat healthily. n Controlling the risk factors

lifestyles n Be physically active. of non-infectious diseases
n Avoid smoking and excessive n Removing the
drinking. sources of infection
n Maintain good personal n Breaking the
hygiene. chain of infection

Receive vaccines n Receive HPV vaccines. n Avoiding the development

n Receive seasonal influenza of diseases
vaccines every year.

Have regular health n Adult women perform a breast n Detecting and treating
check-ups self-examination every diseases at an early stage
month.
n Adults check their
blood pressure, blood
cholesterol and sugar levels in
blood at least once every two
years.

• As a responsible citizen, it is suggested people continue to use insect repellent for some time
after a trip to a country where mosquito-borne diseases are prevalent.
• This acts a precaution to reduce risk of transmission of pathogens to other people by
mosquito bites.

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Supplementary Note

B At community level
n Community health is the protection and improvement of health of the whole
community. Local government has initiated activities to promote community health.
Activity Description

Immunization n Established to protect different target groups from certain

programmes diseases by inducing immunity to a disease, e.g. via vaccine
1) Hong Kong Childhood Immunization Programme, free
vaccines for some serious diseases (e.g. hepatitis B vaccine and
HPV vaccine) are provided to children
2) Seasonal influenza and pneumococcal infections: free or
subsidized vaccines provided to high-risk groups, such as
children, old people*, healthcare workers (high exposure to the
pathogens)
n If majority of people are immunized, the chain of infection can
be broken. ** Outbreaks of diseases can be prevented and the
health of non-immunized individuals can also be protected.
n *old people have a weaker immune system/ are more difficult to
recover from these diseases/ higher risk of outbreaks of these
diseases in elderly’s homes of elderly/ low income and cannot
afford vaccines.

Screening n Involve the identification of a previously undiagnosed disease in

programmes apparently healthy people
n Allow early diagnosis to improve the cure rate and help save
health resources by treating diseases before they progress
n Subsided screening programmes for colorectal cancer, cervical
cancer, breast cancer are offered to different target groups.

Disease surveillance n Involves the identification of the current incidence and trends of
diseases. This helps the health authority carry out timely
investigations and develop effective disease-control strategies
n In HK, the surveillance system is used to collect, analyze, and
report the data of infectious diseases, occupational diseases (e.g.
asbestos-related diseases) and cancer.

Health education n Educates the public about the importance of adopting healthy

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Supplementary Note

lifestyles and taking preventive measures against diseases (e.g.

maintaining good personal and environmental hygiene), and
receiving early treatment when required

Other measures to n Put people who have close contact with the patients in quarantine.
prevent the outbreak n Prohibit people from making unnecessary visits to health care
of diseases facilities (e.g. hospitals)
n Prohibiting group gatherings in public areas.
n Encourage citizens to wear a mask when having symptoms of
respiratory illnesses (reduce droplet transmission), protecting
others in the surrounding and safeguarding community health.

**Notes - Herd immunity

• occurs when a high percentage of the community is immune to a disease through previous
vaccination or/and from previous infection.
• This can effectively stop the spread of disease in the community by making it difficult for
the pathogen to transmit.
• Herd immunity is particularly crucial for protecting people who cannot be vaccinated (e.g.
people with defective immune system)

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Supplementary Note

Exercise
1. Which of the following is not an infectious disease?
A. hepatitis B
B. measles
C. cholera
D. lactose intolerance

2. Which of the following combinations is correct?

Disease Causative agent Preventive measure
A. AIDS virus take contraceptive pill before sexual intercourse
B. cholera bacterium avoid direct contact with infected people
C. malaria protist apply mosquito repellent
D. influenza bacterium wear a surgical face mask if infected

3. What is the role of mosquitoes in the transmission of dengue fever?

A. pathogen
B. host
C. parasite
D. vector

4. Which of the following diseases is/are caused by viruses?

(1) Zika fever
(2) tuberculosis
(3) chickenpox
A. (1) only
B. (2) and (3) only
C. (1) and (3) only
D. (1), (2) and (3)

5. Which of the following contraception method is the most effective in preventing the spread of
syphilis?
A. diaphragms
B. male condoms
C. spermicide
D. vasectomy

6. Which of the following are appropriate preventive measures against hepatitis B?

(1) Do not share needles and syringes for any injections.
(2) Use condom during sexual intercourse.
(3) Avoid direct physical contact with infected people.

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Supplementary Note

A. (1) and (2) only

B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)

7. Which of the following statements about antibiotics is/are correct?

(1) Antibiotics can treat all infectious diseases.
(2) Some antibiotics inhibit the synthesis of bacterial cell walls.
(3) One type of antibiotics can only kill one type of microorganisms.
A. (2) only
B. (3) only
C. (1) and (2) only
D. (2) and (3) only

8. After starting a course of antibiotics, the patient may still feel ill for two or three more days as
A. pathogens are still multiplying in the body.
B. antibiotics cause damage to both pathogens and body tissues.
C. antibiotics can only relieve the symptoms.
D. antibiotics start to kill pathogens after the whole course is completed.

9. Why do scientists continue to search for new antibiotics even many antibiotics are already
available?
A. Antibiotics lost their functions over time.
B. Antibiotic-resistant bacteria have evolved.
C. The treatment of viral infections requires a broad range of antibiotics.
D. Broad-spectrum antibiotics are more effective in killing pathogens.

10. Which of the following about sulpha drugs is/are incorrect?

(1) Sulpha drugs are synthetic chemical compounds.
(2) Sulpha drugs are effective against viral diseases.
(3) Sulpha drugs are enzyme inhibitors.
A. (2) only
B. (3) only
C. (1) and (2) only
D. (2) and (3) only

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Supplementary Note

11. A person was contracted with typhoid (傷寒). Typhoid is caused by the bacterium Salmonella
typhi. It is transmitted by contaminated water or food. The symptoms include a severe fever and
diarrhoea.
(a) Explain why the following symptoms of typhoid may lead to death if the person did not
receive proper treatment.
(i) high fever (3 marks)

(ii) diarrhoea (2 marks)

(b) After contracting typhoid, the person showed no symptoms of the disease for the first few
days. Explain why this is especially dangerous if this person worked in a kitchen.
(2 marks)

(c) Suggest two precautions to prevent the spread of typhoid to other people. (2 marks)

(Total: 9 marks)

12. The graph below shows the prevalence of Methicillin-resistant Staphylococcus aureus (MRSA)
in hospitals between 1996 and 2016. Methicillin is a type of antibiotic.

60
50
40
Prevalence of
MRSA in 30
hospitals (%) 20
10
0
1996 2006 2016
Year

(a) What are antibiotics? (2 marks)

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Supplementary Note

(b) Give one named example of antibiotics. (1 mark)

(c) State two ways by which an antibiotic can affect Staphylococcus aureus. (2 marks)

(d) Suggest an explanation to account for the change in the prevalence of MRSA in hospitals
between 1996 and 2016. (5 marks)

(Total: 10 marks)

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Supplementary Note

1. D 2. C 3. D 4. C 5. B
6. A 7. A 8. A 9. B 10. A
(1 mark each)

11. (a) (i) When a person have a high fever, his/her body temperature increases. (1)
The higher temperature may cause important metabolic enzymes to become denatured,
(1)
hence the normal metabolism of the body is affected. (1)
(ii) Diarrhoea may lead to excess loss of mineral ions together with water. (1)
This would affect the water potential of the blood and tissue fluid. (1)
(b) This person has close contact with food and drinks. (1)
Bacteria can easily contaminate food and drinks during preparation if the person does not
keep good hygiene practice. (1)
(c) Any two of the following: (2)
Ÿ washing hands after going to the toilet and before handling food (1)
Ÿ refrigerating food at the right temperature to inhibit the growth of Salmonella typhi (1)
Ÿ cooking food and boiling water thoroughly before consumption (1)
(Total: 9 marks)

12. (a) Antibiotics are chemicals produced by some microorganisms (1)

to kill or inhibit the growth of other microorganisms. (1)
(b) Penicillin / other acceptable answer (1)
(c) Any two of the following: (2)
Ÿ Inhibit replication of bacterial nucleic acid. (1)
Ÿ Inhibit the synthesis of proteins. (1)
Ÿ Destroy bacterial cell membranes. (1)
Ÿ Inhibit the synthesis of bacterial cell walls. (1)
(d) Methicillin is over-prescribed to treat Staphylococcus aureus. (1)
Genetic variations exist among Staphylococcus aureus in their resistance against
methicillin. (1)
Methicillin only kills non-resistant form while the resistant form survives. (1)

The resistant form continues to reproduce rapidly because of little intraspecific competition.
(1)
The resistant form passes the methicillin-resistant trait to the next generation and
Staphylococcus aureus without the Methicillin-resistant trait are gradually replaced by
those with the resistant trait. (1)
(Total: 10 marks)

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