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Pathogens, The proof that specific bacteria were the cause of :
specific diseases marked the first great step in the rational, scientific
control. of commmicable diseases. The more important members of this
_ group will be cmsidered in connection with the diseases which they cause,

Proof of Disease Production

In’ order to be certain that a given orgenism is the cause of a given
disease, a very exact, scientific method was devised wiich is know as
Koch's Postulates, after their originator:

(a) Same bacteria always found with some disease.
(b) Same bacteria isolated from diseased animal and grown in pure

culture outside the body,
(c) Pure culture must reproduce disease in animal.
(ad) Same organisms recovered from enime1l am grown in pure culture,

Growth and Reproduction of Micro-organisms

The gravth and reproduction of bacteria are dependent upon favorable
conditions adapted to their peculiar needs:

(a) Temperature
(b) Moisture
(c) Rgod supply

bestruction of Micro-Organisms

Sterilization or disinfection is the name applied to the procedure
of destruction of micro-organisms.

Methods: 1, Heat--dry or moist. 2. Sunlight. 35. Chemicals.

Spread of Micro-organisms

A knowledge of the ways by which micro-organisms spread from person
to person is vital to any intelligent defensive hygiene plan against
commmicable diseases. Chapin classifies then as follows:

(a) By cmtact

(b) By fomites (inanimate obj ects)
(c) By air

(a) By food am drink

(e) By insects

Sources of Infection
The chief source of infections is to be found in man and lower animals.
Modes of Infection

The micra-organisms which cause disease enter the body through one
of the following chamels:

(a) Respiratory tract
(b) Digestive tract

(c) Genito-urinary tract
(a) Skin and conjunctiva