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INFECTION
Infection and immunity involve interaction between. the animal body (host) and the
infecting microorganism. Based on their relationship to their hosts, microorganisms can be
classified as saprophytes and parasites. Exceptionally, however, some saprophytes like
B.subtilis may infect devitalized hosts whose natural resistance is greatly reduced
(opportunistic infection).
Parasites are microbes that can establish themselves and multiply in hosts. Parasitic
microbes may be-either pathogens (from Greek pathos suffering, and gen produce, that
is, disease-producing) or commensals (from Latin com with; and mensa table, that is living
together}.
Pathogens are microorganisms that are capable of producing disease in the host. Commensal
microbes live in complete harmony with the host without causing any damage to it. The
normal bacterial flora of the body consists largely of commensals. Many commensals behave
as facultative pathogens in that they can produce disease when the host resistance is
lowered.
The lodgment and multiplication of a parasite in or on the tissues of a host constitutes
infection. It does not invariably result in disease. Disease is a consequence of infection, which
is a common natural event. Infections may be classified in various ways.
 Initial infection with a parasite in a host is termed primary infection.
 Subsequent infections by the same parasite in the host are termed reinfections.
 When a new parasite sets up .an infection in a host whose resistance is lowered by a
preexisting infectious disease, this is termed secondary infection.
 Focal infection (more appropriately focal sepsis) indicates a condition where, due to
infection or sepsis.at localized sites such as the appendix or tonsils, generalized effects
are produced.
 When in a patient already suffering from a disease a new infection is set up from
another
host or another external source, it is termed cross-infection.
 Cross-infections occurring in hospitals are called nosocomial infections (from Greek
nosocomion hospital).
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• The term iatrogenic infection refers to physician induced infections resulting from
investigative, therapeutic or other procedures.
 Depending on whether the source of infection is within or outside the host's own
body,
infections are classified as endogenous or exogenous, respectively. Based on the clinical
effects of infections, they may be classified into different varieties.
 Inapparent infection is one where the clinical effects are not apparent. The term sub clinical
infection is often used as a synonym.
 Atypical infection is one in which the typical or characteristic clinical manifestations of the
particular infectious disease are not present.
Some parasites, following infection, may remain in the tissues in a latent or hidden form,
proliferating and producing clinical disease when the host resistance is lowered. This is
termed latent infection.
SOURCES OF INFECTION
Humans: The commonest source of infection in humans is humans themselves. The parasite
may originate from a patient or a carrier. A carrier is a person who harbors the pathogenic
microorganism without suffering any ill effect because of it. A healthy carrier is one who
harbors the pathogen but has never suffered from the disease caused by the pathogen.
A convalescent carrier is one who has recovered from the disease and continues to harbor the
pathogen in his body. Depending on the duration of carriage, carriers are classified as
temporary and chronic.
The temporary carrier state lasts less than six months. Chronic carnage may last for several years
and sometimes even for the rest of one's life.
The term contact carrier is applied to a person who acquires the pathogen from a patient.
The term paradoxical carrier refers to a carrier who acquires the pathogen from another carrier.
Animals: Many pathogens are able to infect both human beings and animals .Animals may,

Animal Disease therefore, act as sources of human

Cattle Anthrax, Brucellosis ,Tuberculosis infection. In some instance, the infection in

Goat Brucellosis animals may be asymptomatic. Such

Dogs Rabies animals serve to maintain the parasite in

Horse Tetanus,Glanders
Rats Rat bite fever, Weil disease.
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nature and act as the reservoir of human infections. They are, therefore. called reservoir
hosts. Infectious diseases transmitted from animals to human being are called zoonoses.
Zoonotic diseases may be bacterial (plague from rats. viral (rabies from dogs), protozoan
(toxoplasmosis from cats), helminthic (hydatid disease from dogs) or fung-: (zoophilic
dermatophytes from cats and dogs).
Insects: Blood sucking insects may transmit pathogens to human beings. The diseases so
caused are called arthropod-borne diseases. Insects such as mosquitoes. ticks, mites, flies, fleas
and lice that transmit infections are called vectors. Transmission may be mechanical (for
example, transmission of dysentery or typhoid bacilli by the domestic fly). Such vectors are
called mechanical vectors.
In other instances, the pathogen multiplies in the body of the vector, often undergoing part of
its developmental cycle in it. Such vectors are termed biological vectors (for example, Aedes
aegypti mosquito in yellow fever, Anopheles mosquito in malaria). Biological vectors transmit
infection only after the pathogen has multiplied in them sufficiently or has undergone a
developmental cycle. The interval between the time of entry of the pathogen into the vector
and the vector becoming infective is called the extrinsic incubation period.
Besides acting as vectors, some insects may also act as reservoir hosts (for example, ticks in
relapsing fever and spotted fever). Infection is maintained in such insects by trans ovarial or
transstadial passage.
Insects Diseases Soil and water: Some pathogens can
Houseflies Typhoid fever, Cholera, Dysentery survive in the oil for very long periods.
Body Lice Typhus Relapsing fever Spores of tetanus bacilli may remain
Mosquitoes Malaria, Dengue, Chikungunya viable in the soil for several decades
Rat flea Plaque and serve as the source of infection.
Tick Relapsing fever. Fungi (Histoplasma capsulatum, Nocardia
asteroides) and parasites such as
roundworm and hookworm survive in the soil and cause human infection. Water may act as
the source of infection either due to contamination with pathogenic microorganisms (cholera
vibrio, infective hepatitis virus) or due to the presence of aquatic vectors (cyclops in
guineaworm infection).
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Food: Contaminated food may act as a source of infection. The presence of pathogens in food
may be due to external contamination (food poisoning by staphylococcus) or pre-existent
infection in meat or other animal products (salmonellosis).
METHODS OF TRANSMISSION OF INFECTION
Contact: Infection may be acquired by contact, which may be direct or indirect. Sexually
transmitted diseases such as syphilis and gonorrhea illustrate spread by direct contact. The
term contagious disease had been used for diseases transmitted by direct contact, distinct from
infectious disease, signifying all other modes of transmission.
Methods of transmission of infection
 Contact
 Inhalation
 Ingestion
 Inoculation
 Congenital
 Iatrogenic
 Contact Indirect contact may be through the agency of fomites, which are inanimate
objects such as clothing, pencils or toys which may be contaminated by a pathogen
from one person and act as a vehicle for its transmission to another. Pencils shared by
school children may act as fomites in the transmission of diphtheria, and face towels in
trachoma.
• Inhalation: Respiratory infections such as influenza and tuberculosis are transmitted by
inhalation of the pathogen. Such microbes are shed by the patients into the environment,
in secretions from the nose or throat during sneezing, speaking or coughing. Large drops
of such secretions fall to the ground and dry there. Pathogens resistant to drying may
remain viable in the dust and act as sources of infection. Small droplets, under 0.1 mm in
diameter, evaporate immediately to become minute particles or droplet nuclei (usually 1-
10/lm in diameter) which remain suspended in the air for long periods, acting as sources
of infection.
• Ingestion: Intestinal infections are generally acquired(by the ingestion of food or drink
contaminated by pathogens. Infection transmitted by ingestion may be waterborne
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(cholera), Ai foodborne (food poisoning) or handborne (dysentery). The importance of

finger borne transmission is being increasingly recognized, not only in the case of
pathogens entering through the mouth, but also those that enter through the nose and
eyes.
• Inoculation: Pathogens, in some instances, may be inoculated directly into the tissues of
the host. Tetanus spores implanted in deep wounds, rabies virus deposited
subcutaneously by dog bite and arboviruses injected by insect vectors are examples.
Infection by inoculation may be iatrogenic when unsterile syringes and surgical
equipment are employed. Hepatitis Band the human immunodeficiency virus (HIV) may
be transmitted through transfusion of infected blood, or the use of contaminated
syringes and needles, particularly among addicts of injectable drugs.
 Insects: Insects may act as mechanical or· biological vectors of infectious diseases.
 Congenital: Some pathogens are able to cross the placental barrier and infect the
fetus in uterus. This is known as vertical transmission. This may result in abortion,
miscarriage or stillbirth. Live infants may be born with manifestations of a disease, as in
congenital syphilis. Intrauterine infection with the rubella virus, especially in the first
trimester of pregnancy, may interfere with organogenesis and lead to congenital
malformations. Such infections are known as teratogenic infections.
• Iatrogenic and laboratory infections: Infection may sometimes be transmitted during
administration of injections, lumbar puncture and catheterization, if meticulous care in
asepsis is lacking. Modern methods of treatm~t such as exchange transfusion, dialysis
and organ transplant surgery have increased the possibilities for iatrogenic infections.
Laboratory personnel handling infectious material are at risk and special care should be
taken to prevent laboratory infection.
• The outcome of an infection will depend on the interaction between microbial factors
which
predispose to pathogenicity and host factors which contribute to resistance.

FACTORS PREDISPOSING TO MICROBIAL PATHOGENICITY

The terms 'pathogenicity' and 'virulence' refer to the ability of a microbe to produce disease
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or
tissue injury but it is important to make a distinction between them. 'Pathogenicity' is
generally employed to refer to the ability of a microbial species to produce disease, while the
term 'virulence' is applied to the same property in a strain of microorganism. Thus the species
M. tuberculosis or the polio virus is referred to as being pathogenic. The pathogenic species
M .tuberculosis and the polio virus contain strains of varying degrees of virulence including those
which are avirulent, such as the vaccine strains. The virulence of a strain is not constant and
may undergo spontaneous or induced variation. Enhancement of virulence is known as
exaltation and can be demonstrated experimentally by serial passage in susceptible hosts.
Reduction of virulence is known as attenuation and can be ac~ved by passage through
unfavorable hosts, repeated cultures in artificial media, growth in high temperature or in the
presence of weak antiseptics, desiccation or prolonged storage in culture. Virulence is the sum
total of several determinants.
Adhesion: The initial event in the pathogenesis of many infections is the attachment of the
bacteria to body surfaces. This attachment is not a chance event but a specific reaction
between surface receptors on host cells and adhesive structures (ligands) on the surface of
bacteria. These adhesive structures are called adhesins. Adhesins may occur as organized
structures, such as fimbriae or fibrillae and pili, or as colonization factors. This specific adhesin
may account for the tissue tropisms and host specificity exhibited by many pathogens.
Adhesins serve as virulence factors, and loss of adhesins often renders the strain avirulent.
Adhesins are usually made of protein and are antigenic in nature. Specific immunization with
adhesins has been attempted as a method of prophylaxis in some infections, as for instance
against E coli diarrhea in calves and piglets, and gonorrhea in human beings.
Invasiveness: This refers to the ability of a pathogen to spread in the host tissues after
establishing infection. Highly invasive pathogens characteristically produce spreading or
generalized lesions (for example, streptococcal septicemia following wound infection). while
less invasive pathogens cause more localized lesions (for example, staphylococcal abscess).
Some pathogens, though capable of causing serious or even fatal diseases, lack invasiveness
altogether (for example. the tetanus bacillus which remains confined to the site of entry and
produces the disease by elaborating a potent toxin).
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Toxigenicity: Bacteria produce two types of toxins--exotoxins and endotoxins. Exotoxins are
heat labile proteins which are secreted by certain species of bacteria and diffuse readily into
the surrounding medium. They are highly potent in minute amounts and constitute some of
the most poisonous
substances known. One mg of tetanus or botulinum toxin is sufficient to kill more than one
million guinea pigs and it has been estimated that 3 kg of botulinum toxin can kill all the
inhabitants of the world. Treatment of exotoxins with formaldehyde yields toxoids that are
nontoxic but retain the
ability to induce antibodies (antitoxins). They exhibit specific tissue affinities and
pharmacological activities, each toxin producing a typical effect which can be made out by
characteristic clinical manifestations or autopsy appearances. Exotoxins are generally formed
by Gram-positive bacteria but may also be produced by some Gram-negative organisms such
as Shiga's dysentery bacillus, Vibrio cholera and –enterotoxigenic.
Endotoxins are heat stable lipopolysaccharides (LPS) which form an integral part of the cell
wall of Gram-negative bacteria. Their toxicity depends on the lipid component (lipid A). They
are not secreted outside the bacterial cell and are released only by the disintegration of the
cell wall. They cannot be toxoided. They are poor antigens and their toxicity is not completely
neutralized by the homologous antibodies.
They are active only in relatively large doses. They do not exhibit specific pharmacological
activities. All endotoxins, whether isolated from pathogenic or nonpathogenic bacteria,
produce similar effects. Administration of small quantities of endotoxin in susceptible animals
causes an elevation of body temperature manifested within 15 minutes and lasting for several
hours. The pyrogenic effect of fluids used for intravenous administration is usually due to the
presence of endotoxins from contaminant bacteria. Intravenous injections of large doses of
endotoxin and massive Gram-negative septicemias cause endotoxic shock marked by fever,
leucopenia, thrombocytopenia, significant fall in blood pressure, circulatory collapse and
bloody diarrhea leading to death.
Plasmids: Genes coding for some virulence characteristics may be plasmid borne. Examples of
plasmid borne virulence factors are surface antigens responsible for the colonization of
intestinal mucosa by E coli and enterotoxin production by E coli and Staph. aureus. Multiple drug
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resistance (R) plasmids increase the severity of clinical disease by their resistance to antibiotic
therapy.
Bacteriophages: The classical example of phagedirected virulence is seen in diphtheria. In the
diphtheria bacilli, the gene for toxin production is present in beta or other tox+
corynephages.
Communicability: The ability of a parasite to spread from one host to another is known as
communicability. This property does not influence the production of disease in an individual
host but determines the survival and distribution of a parasite in a community. In general,
infections in which the pathogen is shed in secretions, as in respiratory or intestinal diseases,
are highly communicable. In some instances, as in hydrophobia, human infection represents
a dead end, there being an interruption in the spread of the pathogen to other hosts.
Development of epidemic and pandemic diseases requires the pathogen strain to possess
high degrees of virulence and communicability.
Other bacterial products: Some bacterial products other than toxins, though devoid of intrinsic
toxicity, may contribute to virulence by inhibiting the mechanisms of host resistance.
Pathogenic staphylococci produce a thrombin-like enzyme coagulase which prevents phagocytosis
by forming a fibrin barrier around the bacteria and walling off the lesion. Many pathogens produce
hemolysins capable of destroying erythrocytes. Bacterial appendages: Capsulated bacteria such as
pneumococci, K pneumoniae and H
injluenzae are not readily phagocytosed. Some bacterial surface antigens such as the Vi antigen of S
typhi and K antigens of E coli also help the bacteria to withstand phagocytosis and the lytid activity of
complements. Infecting dose: Successful infections require that an adequate number of bacteria
should gain entry into the host. The dosage may be estimated as the minimum infecting dose (MID)
or minimum lethal dose (MLD) which are, respectively, the minimum number of bacteria required to
produce clinical evidence of infection or death, respectively, in a susceptible animal under standard
conditions. As animals exhibit considerable individual variation in susceptibility, these doses are
more correctly estimated as statistical expressions, ID 50 and LD
50, as the dose required to infect or kill 50 per cent of the animals tested under standard conditions.
Route of infection: Some bacteria, such as streptococci, can initiate infection whatever be the mode
of entry. Others can survive and multiply only when introduced by the optimal routes. Cholera
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vibrios are infective orally but are unable to cause infection when introduced subcutaneously. This
difference is probably related to the modes by which different bacteria are able to initiate tissue
damage and establish themselves. Bacteria also differ in their sites of election in the host body after
introduction into tissues. They also differ in the ability to
damage different organs in different species of animals. Tubercle bacilli injected into rabbits cause
lesions mainly in the kidneys and infrequently in the liver and spleen, but in guinea pigs the lesions
are mainly in the liver and spleen, the kidneys being spared. The reasons for such selective
multiplication in tissues are largely obscure, though they may be related to the presence in tissues
of substances that may selectively hinder or favour their multiplication.
TYPES OF INFECTIOUS DISEASES
Infectious diseases may be localised or generalised. Localised infections may be superficial or deep-
seated.
Generalised infection involves the spread of the infecting agent from the site of entry by
contiguity, through tissue spaces or channels, along the lymphatics or through the
bloodstream. Circulation of bacteria in the blood is known as bacteremia. Transient
bacteremia is a frequent event even in healthy individuals and may occur during chewing,
brushing of teeth or straining at stools. The bacteria are immediately mopped up by
phagocytic cell and are unable to initiate infection. Bacteremia of greater severity and longer
duration is seen during generalised infections as in typhoid fever. Septicemia is the condition
where bacteria circulate and multiply in the blood, form toxic products and cause high,
swinging type of fever. Pyemia is a condition where pyogenic bacteria produce
septicemia with multiple abscesses in the internal organs such as the spleen, liver and
kidneys.
Depending on their spread in the community, infectious diseases may be classified into
different types.
 Endemic diseases are those which are constantly present in a particular area. Typhoid fever
is endemic in most parts of India.
 An epidemic disease is one that spreads rapidly, involving many persons in an area at the
same time. Influenza causes annual winter epidemics in cold countries.
 A pandemic is an epidemic that spreads through many areas of the world involving very
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large numbers of people within a short period. Influenza, cholera, plague and enteroviral
conjunctivitis are pandemic diseases.
 Epidemics vary in the rapidity of spread.
Waterborne diseases such as cholera and hepatitis may cause explosive outbreaks, while
diseases which spread by person-to-person contact evolve more slowly. Such creeping or
smouldering epidemics, as that of cerebrospinal fever, are termed prosodemic diseases.

Exotoxins
Proteins Endotoxins
Heat labile Lipopolysaccharides
Actively secreted by cells; diffuse Heat stable
into surrounding medium Form part of cell wall; do not diffuse into
Readily separable from cultures Qy surrounding
physical means such as ftltration medium
Action often enzymic Obtained only by cell lysis
Specific pharmacological effect for each
exotoxin Specific tissue affmities No enzymic action
Active in very minute doses Effect nonspecific; action common to all
Highly antigenic endotoxins
Action specifically neutralised by antibody No specific tissue affmity
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Active only in very large Neutralisation by antibody

doses ineffective
Weakly antigenic
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