Different sources of infection:
There are many sources of infection such as reservoirs, carrier organisms and lifeless objects.
Reservoirs: A habitat where the disease causing organism lives prior to infection is known as the reservoir for infectious agent. There are three types of reservoirs - human reservoirs, animal reservoirs and reservoir of non-living substances. In the reservoir, the infectious agent lives and even mulitplies. The disease causing organism may or may not produce sickness in the reservoir organisms.
Carrier organisms: Carrier organisms are those in which the disease causing organisms are present in large numbers but does not produce sickness. As a result, carrier organisms live along with normal persons and may even spread disease from person to person. The carrier appears normal but is infected with a disease-causing organism. For example rats are carriers for plague, bacteria; monkeys arecarriers for yellow fever and human beings are carriers for viral diseases such as hepatitis.
Lifeless objects: Some of the lifeless objects such as towels, clothes, injection needles and excreta or sputum of the sick persons may also act as reservoirs for diesease causing organisms. These organisms cannot travel by themselves from the reservoir to a human being. They require some agent to transport them.