Health Hygiene & Sanitation

Lesson 10 : Immunization To Prevent Communicable Disease

Susceptible Host

Susceptibility of the host is another factor determining the transmission of disease causing agents. Sequence of events occurring in the host includes-

1. Portal of entry: The agent can find entry through respiratory tract, alimentary canal, genito-urinary tract, skin etc. Some organisms may have multiple ways of entry.


2. Site selection: Once entered the organism searches for appropriate site or tissue for multiplication and survival.


3. Portal exit: The agent must find a way out of the body and reach a new host. If there is no way out, the infection becomes dead end infection, as in rabies.


4. Survival of organism: The disease causing agent must survive in the external environment till it finds a new host.

The infection becomes apparent only after a period of incubation. Period of infection is defined as “the time interval between invasion by an infectious agent and appearance of the first sign and symptom of the disease in question”. The agent multiplies in the host and when it reaches sufficient density, health equilibrium is disturbed and disease appears.