Infectious diseases can be spread by fol­lowing ways:

(a) By direct contact

(b) Through some medium

(a) By direct contact


Microorgan­isms travel from infected person to other person by sharing food in the same utensil, using the clothes of the patients, etc.

(b) Through medium

Microorgan­isms reach other people through various medium.

(i) Through air


When the patient coughs spits or excretes, many microorgan­isms spread in the air and through the breath­ing they reach other persons and make them infected.

(ii) Through contaminated food and water

Many diseases are spread due to con­taminated water and food. Flies sit on open foodstuffs and contaminate them by leaving microorganisms on them. When this food is consumed by people they get infected and dis­eases like diarrhea, dysentery, etc. are caused.

(iii) Through insects


Mosquitoes carry many microorganisms with them. When they
bite a healthy person they leave these mi­crobes in his blood and make him patient of malaria, dengue fever, etc.

(iv) Through carriers

Sometimes people have the microbes in them but they themselves are not patients but just carriers, i.e., they can infect other people by these mi­croorganisms.

Infectious diseases have a definite period of their own. After that period, the disease dies out naturally. Once the person suffers from such disease, these microorganisms create immunity in the body of the patient for that particular disease. That is why there is very little possibility of the same disease happening again in his lifetime. Time span of the disease can be divided as under:


(i) Incubation period

This is the 1st stage of disease. In this stage, microorganism start growing and producing their toxins, but in this stage no external symptoms of the disease are visible. Every disease has its own incubation period.

(ii) Infection period

When the symp­toms are visible in the body of the patient this is called infection period. It is divided into various stages:


(a) Onset of infection

(b) Period of climax

(c) Period of decline

(d) Period of convalescence


(iii) Relapse

Sometimes the patient is so weak that he cannot take care of himself and is not properly cured. A microorganism again attacks the patient and makes him sick. This situation is very unfortunate as it can leads to grave complications. The patient may not sometime able to sustain it.