The host and environment are in constant interaction and that a disease is caused by disturbance of equilibrium between agent, host and environment. The disease assumes epidemiological proportions when the environmental conditions are favourable for the disease agent and unfavourable conditions exist for man. You must have observed disasters like wars, famine, floods and earthquakes are follow by epidemics of infectious disease.
Why does this happen? It happens because after the disaster, the favorable conditions for occurrence of an epidemic sets in there is no specific or a particular cause, which is responsible for occurrence of epidemic, but various factors complementing and supplementing each other are responsible for occurrence of epidemics.
The following factors favor occurrence of epidemics after disasters.
i) Temporary Population Settlements
Rehabilitation operations that follow a disaster are usually set up in crowded temporary camps or settlements. Provision of safe drinking water, sanitation and other basic services often lack at these places. This results in a rise in the incidence of infectious diseases like dysentery, measles, whooping cough, tuberculosis, scabbies and other skin diseases.
ii) Pre-existent Diseases in the Population
The diseases already occurring in the area are most likely to emerge as epidemics when the area is struck by a disaster. An epidemic of non-existent disease in that area is unlike to be seen after such disasters.
iii) Ecological Changes
During natural disaster like floods and cyclones, ecological changes occur. It causes increase in the breeding sites for mosquitoes. This results in an increase in the cases of malaria. Open defecating and decay and decomposition of organic material increases insect breeding and thereby increases the transmission of disease like conjunctivitis, diarrhea, dysentery, enter virus infections, and parasitic diseases.
iv) Resistance Potential of the Host
The nutritional and immunization status of the host population determines to a large extent its susceptibility to communicable disease. Children with poor nutrition are more likely to get infected with communicable disease and the incidence of measles, whooping cough, diphtheria and tuberculosis is likely to be higher if they are not immunized earlier.
v) Damage to Public Utility and Interruption of Public Health Services
Public utility services like water supply and sewage if damaged may cause large-scale contamination and subsequent introduction of diseases in the population. Interruption of ongoing health programmes in the area may also lead to resurgence of diseases.