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Cambridge (CIE) AS Biology Your notes

Infectious Diseases
Contents
Infectious Diseases
Transmission of Disease
Prevention & Control of Disease

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Infectious Diseases
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Infectious Diseases
A disease is an illness or disorder of the body or mind that leads to poor health
Infectious diseases are caused by pathogens and are transmissible, meaning that they can be spread
between individuals within a population

Infectious & non-infectious disease table

Term Definition Example

Infectious, or Diseases caused by pathogens that can be Cholera

communicable, disease transmitted from infected to uninfected people.
Tuberculosis (TB)
Some can transmit from animals to people, or people
to animals Malaria
HIV/AIDS

Non-infectious disease Diseases that are not caused by pathogens, but that Lung cancer
may instead be inherited, or related to lifestyle factors
or life events Sickle cell
disease
Depression
Coronary heart
disease

Common Pathogens & Their Diseases

Examples of infectious diseases of worldwide importance are
Cholera
Malaria
Tuberculosis (TB)
HIV/ AIDS
The number of people infected with these diseases across the world remains very high, and they pose
serious public health problems

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These diseases are all caused by pathogens; understanding the pathogen and its method of
transmission are important in reducing the spread of disease
Common infectious diseases table Your notes

Disease Pathogen Type of

organism

Cholera Vibrio cholerae Bacterium

Malaria Plasmodium falciparum, Plasmodium malariae, Plasmodium ovale, Protist

Plasmodium vivax

TB Mycobacterium tuberculosis, Mycobacterium bovis Bacterium

HIV/AIDS Human immunodeficiency virus (HIV) Virus

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Transmission of Disease
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Transmission of Common Diseases
Transmission of cholera
Cholera is caused by the bacterium Vibrio cholerae
The disease is water-borne and food-borne
Infected people egest large numbers of the pathogenic bacteria in their faeces and water becomes
contaminated
Cholera can then be transmitted when individuals wash in contaminated water, drink contaminated
water, or eat food exposed to contaminated water
This means the disease occurs where people do not have access to proper sanitation and
uncontaminated food

Transmission of malaria
Malaria is caused by one of four species of the protoctist Plasmodium
These protoctists are transmitted to humans by an insect vector as follows:
Female Anopheles mosquitoes feed on human blood to obtain the protein they need to develop
their eggs
If the person they bite is infected with Plasmodium, the mosquito will take up some of the
pathogen with the blood meal
When feeding on the next human, Plasmodium pass from the mosquito to the new human’s blood
Malaria may also be transmitted during blood transfusion and when unsterile needles are re-used
Plasmodium can also pass from mother to child across the placenta

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Your notes

Plasmodium spends part of its life cycle inside humans and part inside mosquitoes
Transmission of tuberculosis (TB)
TB is cause by the bacterial pathogens Mycobacterium tuberculosis and Mycobacterium bovis
Human to human transmission occurs by droplet infection as follows
When infected people with the active form of the disease cough or sneeze, M. tuberculosis
bacteria enter the air in tiny droplets of liquid
TB is transmitted when uninfected people inhale these droplets
TB therefore spreads more quickly among people living in overcrowded conditions
The form of TB caused by M. bovis occurs in cattle but can spread to humans through contaminated
meat and unpasteurised milk
Very few people in developed countries now acquire TB in this way, although meat and milk can still
be a source of infection in some developing countries

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Transmission of HIV/AIDS
Human Immunodeficiency Virus (HIV) can infect human cells and eventually lead to AIDS Your notes
The virus is spread by intimate human contact and can only be transmitted by direct exchange of
body fluids
This means HIV can be transmitted in the following ways:
Sexual intercourse
Blood donation
Sharing of needles used by intravenous drug users
From mother to child across the placenta
Mixing of blood between mother and child during birth
From mother to child through breast milk

Cholera, malaria, TB & HIV/AIDS summary table

Disease Pathogen Transmission Symptoms

Cholera Vibrio cholerae Contaminated Diarrhoea and dehydration

water or food

Malaria Plasmodium falciparum, P. Female Anopheles Fever, headaches and muscle pain
malariae, P. ovale, P. vivax mosquito

TB Mycobacterium tuberlulosis, M. Airborne droplets Cough, chest pain, fever and

bovis weight loss

HIV/AIDS Human Immunodeficiency Virus Exchange of body Initial flu-like symptoms, and
fluids eventually a loss of immune
function (AIDS)

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Prevention & Control of Disease

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Factors for Prevention & Control of Common Diseases
Prevention & control of cholera
Cholera occurs when people do not have access to effective sanitation facilities and access to clean
water
It is difficult to prevent and control cholera in developing countries
The fast-growing cities in developing countries may not have the appropriate infrastructure, and
may have limited funds for large-scale projects such as the provision of drainage systems, sewage
treatment facilities and clean water supplies
Humanitarian crises, e.g. displacement of people due to wars or natural disasters, can cause the
destruction of sanitation infrastructure and/or the provision of poor sanitation facilities in
overcrowded temporary housing
In some developing countries the use of raw human sewage to irrigate crops is common
Prevention of cholera can occur through:
Providing adequate sewage treatment infrastructure
The provision of clean, piped water that has been chlorinated to kill bacteria
This strategy means that cholera is very rare in developed countries
Vaccination programmes in areas where cholera is common
Cholera can be controlled by:
Ready access to treatments such as oral rehydration therapy; a solution containing glucose, salts
and water
Monitoring programmes by the World Health Organisation (WHO)
Using antibiotics in severe cases

Prevention & control of malaria

Malaria spreads in regions where Anopheles mosquitos can breed; this is dependent on a warm
climate, and the availability of standing water
The 3 main methods for reducing malaria are:
Reducing the number of Anopheles mosquitoes in an area

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Spraying living areas with insecticides

Spreading oil over the surface of water bodies in which mosquitoes breed Your notes
Draining marshes and other unnecessary bodies of water
Ensuring ponds and irrigation or drainage ditches are stocked with fish that feed on mosquito
larvae
Unfortunately, mosquitoes lay eggs in even very small puddles and pools of water and
therefore it is practically impossible to control all breeding sites using the methods listed
above
Reducing the chance of being bitten by these mosquitoes
People in malarial zones should sleep under bed nets, which can also be soaked periodically in
insecticide to increase effectiveness,
People should avoid exposing their skin at dusk when mosquitoes are most active
Using drugs to prevent Plasmodium infecting humans
Drugs, such as chloroquine and mefloquine, are taken before, during and after a visit to a
location where malaria is prevalent.
The use of these drugs has resulted in drug-resistant strains of Plasmodium
The drugs are expensive and can have disagreeable side-effects
In the 1950s, the World Health Organisation (WHO) coordinated a worldwide eradication programme;
whilst malaria was eradicated from some countries, the programme was mainly unsuccessful because:
Plasmodium became resistant to the drugs being used to try and control it
Anopheles mosquitoes became resistant to DDT and other insecticides being used against them
To control malaria, governments, the WHO and other institutions are focusing on:
Working within health systems to improve diagnosis
Improving the supply of effective drugs
Using drugs in combination to reduce drug resistance
Promoting appropriate methods to prevent transmission, e.g. the use of biological controls to
target the larvae and insecticide-treated bednets
Recent scientific advances regarding the control of malaria are:
Simple dipstick tests for diagnosing malaria – this means a diagnosis can be made much faster and
does not require a laboratory
The entire Plasmodium genome has been sequenced

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The approval of two effective malaria vaccines which have been decades in development; these
can now be rolled out in affected areas
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Malaria occurs in regions where the climate is suitable for the Anopheles mosquito
Prevention & control of tuberculosis (TB) Your notes

TB is spread from person to person when droplets released by the coughing or sneezing of an infected
person are inhaled by an uninfected person
The droplets contain the TB-causing bacterium Mycobacterium tuberculosis
The spread of TB increases in overcrowded living conditions, so is prevalent among poorer people
with inadequate housing conditions
Contact tracing, and the subsequent testing of those contacts for the bacterium, is an important part
of controlling the spread of TB
Prevention of TB occurs through the use of the BCG vaccine
The vaccine protects up to 70-80% of those who receive it, although its effectiveness decreases
with age
The form of TB that can be transmitted between cattle and humans (caused by Mycobacterium bovis)
can be prevented by:
Routinely testing cattle for TB and destroying those that test positive
Pasteurising milk; this kills any TB-causing bacteria present in the milk
Ensuring that meat is cooked properly

Prevention & control of HIV/AIDS

HIV is spread when body fluids are exchanged between an infected and an uninfected individual
Preventing the spread of HIV is very difficult as the virus has a long latent stage, which results in it being
transmitted by people who have the virus but show no symptoms, and so may not know they are
infected
This occurs because the virus can change its surface proteins, making it difficult for the human
immune system to recognise it and for a vaccine to be developed
To prevent the transmission of HIV the following measures can be implemented:
Blood donations can be screened for HIV and heat-treated to kill any viruses
HIV-positive mothers and their babies can be treated with drugs
Condoms, femidoms and dental dams can be used to decrease the infection risk during sexual
intercourse and oral sex by forming a physical barrier between body and fluids

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Education programmes about how the virus is transmitted can be released into the community
to encourage people to have HIV tests and to avoid unprotected sex
Your notes
Intravenous drug users encouraged not to share needles
Controlling HIV can occur by:
Contact tracing (and the subsequent testing of those contacts for the virus)
Screening blood donations
Public health measures, such as widespread HIV testing of the population and education
programmes
Needle-exchange schemes have been set up in some places to exchange used needles for new,
sterile ones
Encouraging individuals to be tested for HIV
Using anti-retroviral drugs
The socio-economic status of a person or country with HIV can determine how it is controlled
For example, HIV-positive mothers are advised not to breastfeed in high-income countries,
however, in low- and middle-income countries breastfeeding is more affordable, and offers
protection against other diseases, e.g. cholera

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