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Introduction
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General characteristics of infectious agents
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Factors in host affecting the infections
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Methods of identification
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Important bacterial diseases
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Important viral diseases
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Important fungal diseases
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Infection: The invasion and multiplication of
microorganisms such as bacteria, viruses, and parasites
that are not normally present within the body.
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Microorganisms [bacteria, viruses, fungi and parasites] are
present everywhere – soil, water, air, body surfaces etc.
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Few microorganisms are beneficial while others are
responsible for large number of infectious diseases.
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vaccines and medications have been successful in
controlling and eliminating some diseases but still
infections are a major cause of death.
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This could be due to –
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Development of newer and resistant strains of microrganisms
Immunosuppressive therapy in malignant and transplant patients
Hospital acquired infections – nosocomial infections
AIDS
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Host –organism interrelationship –
› Symbiosis: beneficial to both.
› Eg:- bacteria
› Commensalism: benefitting one but not harming
the other
› True parasitism: benefitting the parasite but
harming the host.
› Saprophytism: organisms living on dead tissue.
1. Mode of entry:
› Ingestion: eating
› Inoculation: parenteral method
› Inhalation: respiration
› Perinatally: from mother before birth.
› Direct contact
› From animal host
2. Spread of infections: may spread through
phagocytic cells, blood vessels and lymphatics.
3. Production of toxins: many bacteria produce toxins
which cause pathologic effect.
endotoxin – liberated on lysis of bacterial cell
exotoxin – secreted by bacteria, have effects
on different sites as well.
4. Virulence of organism: ( capacity of a
microorganism to cause disease) many organism
have different species and strains. C.diphtheriae
(gravis, intermedius & mitis)
5. Product of organism: some of them produce
enzymes which help in its spread. Ex - coagulase by
staphylococci and hyaluronidase by Streptococcus
pyogenes
1.Physical barrier: break in continuity of skin or
mucous membrane.
2. Chemical barrier: secretions of the oral cavity, GI
tract.
3. Effective drainage: respiratory system, salivary
gland urinary system all have to be drained
effectively if there is any blockage this leads to
infection.
4. Immune defense mechanism: very important that
all branches of the immune system should be
functioning properly.
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In addition to theses techniques, culture of lesional tissue should
be carried out for drug sensitivity.
Organisms are seen at the advancing edge of the lesion rather
than the center.
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Enteric fever: acute infection of salmonella typhi
or paratyphi.
Mode of entry – ingestion through contaminated
food.
Pathogenesis – initially get localized to lymphoid
follicles and proliferate (asymptomatic phase).
Next they enter the blood stream (bacteraemia)
to cause rise in temperature and ‘rose spots’
finally get localized to intestinal lymphoid tissue,
liver, gall bladder and spleen.
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Food poisoning: this is an acute bacterial illness
caused by ingestion of contaminated food or water.
This excludes the specific diseases like typhoid or
cholera.
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Types:
› Staphylococcal food poisoning: common type, usually
causes mild stomach pain and discomfort. Severe form of
this infection is known as psedo-membranous enterocolitis.
› Clostridial food poisoning: caused by cl.welchii, usually
from contaminated meat products.
› Botulism: severe form of poisoning caused by cl.botulinum.
Results in paralysis due to production of neurotoxin.
› Salmonella food poisoning: caused by s.typhimurium.
Fever, vomiting & diarrhoea
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Anthrax: bacterial disease caused by bacillus anthracis.
It spreads from animals to man. Widely prevalent in sheep
and cattles. very rare in humans but has gained importance
because of possible use in ‘bio-terrorism’
it is a spore forming bacteria and the spores are very resistant.
Depending on the mode of entry three types of human anthrax
are seen
Cutaneous : most common, direct contact with skin [Hide
bound disease].
Pulmonary : inhalation, most fatal “wool sorters” disease.
Gastrointestinal: by ingestion, very rare.
Haemorrahage, oedema and necrosis is seen at the point of
entry.
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Tetanus: it is disease caused by clostridium tetani
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Mode of entry: wound contamination and often involves
a cut or puncture wound
Pathogenesis:- The incubation period of tetanus
may be up to several months, but is usually about
eight days
The first sign is trismus, or lockjaw, and the facial
spasms called risus sardonicus , followed by stiffness of
the neck, difficulty in swallowing, and rigidity of
pectoral and calf muscles. Other symptoms include
elevated temperature, sweating, elevated blood
pressure , and episodic rapid heart rate
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The "spatula test" is a clinical test for tetanus
that involves touching the posterior
pharyngeal wall with a soft-tipped
instrument and observing the effect
 Tetanus can be prevented
by vaccination with tetanus toxoid
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Treatment: Tetanus toxoid can be given in
case of a suspected exposure to tetanus.
 Sedatives
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Many fungal species have been discovered till now
but only a few are infective to humans.
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Many of the fungal infections are opportunistic
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Some common examples of fungal diseases are
› Candidiasis
› Mycetoma
› Aspergillosis
› Histoplasmosis
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Candidiasis: opportunistic infection caused
by candida albicans
This organism is usually present as normal flora of
the skin and mucous membranes, but it
becomes pathogenic when balance between
host and organism is disturbed.
Types
› Oral candidiasis – thrush, atropic, angular
chelitis etc
› Candidal vaginitis
› Cutaneous candidiasis
› Systemic candidiasis
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Aspergillosis: aspergillus fumigatus
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it is the most common fungal infection of the lung.
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Grows best in cool wet climate
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May cause allergic broncho pulmonary
aspergillosis
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In immunocompromised patient may cause serious
disease
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hematogenous spread may cause extensive
lesions.
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Viral diseases are the most common cause of
human illness.
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A single virus may cause different diseases in the
same host depending on the immune status and
age of the patient. Eg - varicella zoster virus.
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Viruses are essentially intracellular parasites.
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They may be single stranded or double stranded.
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RNA or DNA virus
Yellow fever – oldest known haemorrhagic fever.
 Monkeys carry the virus without being affected.
Transmitted by mosquito – aedes aegypti.
 Fever, chills, hepatic failure, renal failure, bleeding
disorders.
Dengue fever - aedes aegypti.
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break bone fever
Other viral infections – herpes group, mumps, HIV,
Rabies virus. etc
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Two main groups protozoal and helminthic group.
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Parasitology
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These diseases are quite common.
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They may cause disease due to their presence in blood
stream, intestine, inside the cells etc.
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Some of the common diseases are –
› Amoebiasis
› Malaria
› Filariasis
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References –
› Essential pathology for dental students –
Harsh Mohan
› Robbins basic pathology – Kumar, Abbas
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