Communicable diseases spread from the infected person to a healthy person.

Direct Transmission

The pathogens of diseases infect a healthy person directly without an intermediate agent. It can take place by various means such as:

(i) Direct contact between the infected person and the healthy person: Diseases like small pox, chicken pox, syphilis, gonorrhoea spread through direct contact.

(ii) Droplet infection: The infected person throws out tiny droplets of mucus by coughing, sneezing or spitting. These droplets may contain the pathogen. By inhaling the air containing the droplets, a healthy person may get the infection.

Diseases like common cold, pneumonia, influenza, measles, tuberculosis and whooping cough spread through droplet infection.

(iii) Contact with soil contaminated with disease-causing viruses and bacteria.

(iv) Animal bite: Viruses of rabies are introduced through the wound caused by the bite of rabid animals, especially dogs. The virus is present in the saliva of the rabid animals.

Indirect Transmission

The pathogens of certain diseases reach the human body through some intermediate agents. It can take place by various means:

(i) By vectors such as houseflies, mosquitoes, and cockroaches. Examples: Houseflies carry the causative organisms of cholera on their legs and mouth parts from the faeces and sputum of infected persons to food and drinks and contaminate them. When this contaminated food is taken by a healthy person, he gets the infection. Similarly, mosquitoes carry virus of dengue and malarial parasite which causes malaria.

(ii) Air-borne: The pathogens may reach humans with air and dust. The epidemic typhus spreads by inhalation of dried faeces of infected fly.

(iii) Object borne (Fonite borne): Many diseases are transmitted through the use of contaminated articles, such as clothes, utensils, toys, door handles, taps, syringes and surgical instruments.

(iv) Water borne: If potable water (drinking water) is contaminated with pathogens of diseases such as cholera, diarhhoea, hepatitis or jaundice, it reaches a healthy person upon consuming such water.