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Sources of Infection

A. Endogenous infection

  1. coli, E. faecalis: UTI;
  2. Viridans streptococci: infective endocarditis 

B. Exogenous infections

Human cases & carriers

  1. Cases :  AIDS, syphilis, TB
  2. Carriers :  Harbors the pathogenic microorganisms without suffering from it
  3. Healthy :  Harbors but never suffered from the disease
  4. Convalescent :  Recovered from disease but harbors the pathogen
  5. Temporary :  Less than six months
  6. Chronic :  More than one year
  7. Paradoxical :  Who acquires the pathogen from another carrier
  8. Contact :  Who acquires pathogen from a patient. 

2.  Animal cases and carriers:

Contact with animal, animal bite, ingestion of milk & meat; Zoonoses

  1. Bacterial :  bovine tuberculosis, bubonic plague, anthrax
  2. Viral :  rabies
  3. Helminthic :  hydatid disease
  4. Fungal :  Microsporum canis

Man generally acts as a dead end host in such diseases. Exception: pneumonic plague


3.  Insects: called as vectors

  1. Mechanical vectors: carry the organisms on their legs, wings, and body. Transmit them onto eatables, which act as source of infection e.g.: salmonellosis & shigellosis by domestic fly
  2. Biological vectors: those in which pathogen multiplies or undergoes developmental changes with or without multiplication
    1. Propagative: when pathogen only multiplies: plague bacilli in rat fleas
    2. Cyclo-propagative: pathogen undergoes developmental change and multiplies: malarial parasite in female anopheles mosquito.
    3. Cyclo-developmental: pathogen undergoes development without multiplication: filarial parasite in culex mosquito.
    4. Extrinsic incubation period: time required for the biological vector to become infective from the time of entry of the pathogen into it.

4.  Environment:  soil, water, food

  1. Soil: tetanus, gas gangrene, mycetoma
  2. Water: shigella, salmonella, vibrio cholerae, polo virus, hepatitis A virus
  3. Food: organisms causing food poisoning, diarrhoea, dysentery

C.     Organism Spreading Infection


Diseases transmitted

1. Mosquito


a. Anopheles


b. Culex

Bancroftian filariasis, Japanese encephalitis, West Nile fever

c. Aedes

Yellow fever, Dengue, Chikungunya fever

d. Mansonia

Brugian filariasis


2. Flies


a. House fly

Enteric fever, diarrhoea, dysentery, cholera, amoebiasis

b. Sand fly

Kala-azar, oriental sore, espundia, oroya fever, sandfly fever

c. Tse-tse fly

African trypanosomiasis

3. Louse

Epidemic typhus, epidemic relapsing fever, trench fever

4. Rat flea

Bubonic plague, endemic typhus, Hymenolepis diminuta infection

5. Black fly (simulium spp.)


6. Deer fly (chrysops spp.)

Loa Loa

7. Reduviid bug

American trypanosomiasis (Chaga’s disease)

8. Ticks


a. Hard tick

Spotted fever group, tularemia, babesiosis, Lyme’s disease, Colorado tick fever, ehrlichiosis

b. Soft tick

Q fever (in animals), endemic relapsing fever, KFD

9. Mite


a. Trombiculid mite

Scrub typhus

b. Gamasid mite

Rickettsial pox

10. Cyclops

Dracunculiasis, diphyllobothriasis


Microbial Pathogenicity

  1. Capacity of an organism to initiate disease.
  2. It requires the following attributes:
    1. Transmissibility: communicability from one host to a fresh host
    2. Infectivity: ability to breach the new host’s defenses
    3. Virulence: capacity of the pathogen to harm the host

Determinants of virulence:

  1. Adhesion:
    1. Fimbriae, responsible for tissue tropism important in certain bacteria like E. coli, N. gonorrhoeae
    2.  Teichoic acids: Streptococci, Staphylococci
  2. Avoidance of host defence mechanisms:
    1. Capsules: inhibits phagocytosis. H. influenzae, S. pneumoniae, N. meningitides
    2. Streptococcal M protein: inhibits phagocytosis

Resistance to killing by phagocytic cells:

  1. M. tuberculosis: inhibits phagolysosome fusion
  2. S. aureus & N gonorrhoeae: catalase negates the effects of toxic oxygen radicals
  3. Antigenic variation: Trypanosoma brucei, N. gonorrhoeae, B. recurrentis
  4. IgA1 proteases: N. meningitides, H. influenzae, Streptococcus pneumoniae
  5. Serum resistance: Able to resist lysis due to deposition of compliment; smooth strains resistant than rough strains.
  6. Toxins: substances produced or present in bacteria, which have direct toxic action on the tissue cells. Two main types viz. exotoxins & endotoxins.

D.    Difference between Exotoxins & Endotoxins





Heat labile

Heat Stable

Highly antigenic

Weakly antigenic

Actively secreted by the cells

Integral part of cell wall

Can be converted into toxoid

Cannot be converted into toxoid

Acts on specific receptors, different exotoxins have different actions

All endotoxins have similar non-specific action

High potency

Low potency

Produced by gram positive & negative bacteria

Produced by gram negative bacteria only

Frequently coded by plasmids

Coded by chromosomal genes


1.  Exotoxins

  1. Having lethal action
  2. C. Vbotulinum toxin A: Neuromuscular junction
  3. Tetanus toxin: Voluntary muscle
  4. Diphtheria toxin: Heart
  5. Pyrogenic effect: toxic shock syndrome toxin of S. aureus, S. pyogenes (super antigens)
  6. Enterotoxin: Cholera enterotoxin, LT & ST of E. coli, C. difficile, C. perfringens, B. cereus
  7. Toxins acting on skin: Epidermolytic toxin of S. aureus, erythrogenic toxin of S. pyogenes 

2.  Special Points

  1. Extracellular enzymes: Urease, hyluronidase, coagulase, collagenase, stretokinase    
  2. Toxins which act by ADP-ribosylation- Diphtheria toxin, LT of ETEC, Pertussis toxin, Cholera toxin
  3. Toxins which act by increasing cAMP- CT, LT of ETEC, edema factor of B. anthracis toxin, Pertussis toxin
  4. Toxin which act by increasing cGMP- ST of ETEC
  5. Toxins which act by inhibition of protein synthesis-Shiga toxin, Verotoxin, Diphtheria toxin

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