SOURCES OF INFECTION

1. Human sources
(a) Patients in the incubation period or during the course or convalescent stage of an infection

(b) Carriers
             Healthy carriers are person in good health who harbour pathogens such as Strep. pyogenes, Staph. pyogenes C. diphtheriae or N. meningitidis.
Convalescent carriers are persons who harbour the causal organism, sometimes for many years, after clinical recovery from a disease, e.g. typhoid bacilli have been recovered from a carrier 25 years after an attack of typhoid fever.

(c) Autoinfection. From the time that the feotus enters the maternal birth canal, all individuals tend to accumulate a similar symbiotic flora, among which are many potential pathogens. Various factors which lead to autoinfection are ecognised. Ageneral lowering of resistance enables micro-organisms such as the virus of herpes simplex or the fungus of thrush to gain a foot hold. Antibiotic therapy, which interferes with ecological conditions, may ensue. Thus haemolytic streptococci, derived from the upper respiratory tract, may enter the blood stream from the mouth and cause subacute bacterial endocarditis, while Esch. coli may pass up the female urethra and infect the urinary tract.

2. Animal Reservoirs. Diseases of animals which may be acquired by man include psittacosis, acute lymphocytic choriomeningitis, Q fever, bovine tuberculosis, leptospirosis and helminth infections in which the organism enters the body through one of the natural routes.

3. Soil. The organisms causing tetanus, gas gangrene and botulism are normally found in cultivated soil.

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