Source or Reservoir of Infections
A reservoir is any animate or inanimate substance in which an infectious agent multiplies or develops and upon which it depends as a species for its survival in nature.
Sources of Infections
1. Cases - actually suffering from a disease and shed pathogens.
2. Carriers - infected and shedding pathogens without showing any clinical signs. These could be healthy, incubating, convalescent, chronic, transient or latent carriers.
Carriers are more important in populations than are cases because the infectious agent is maintained for a long time and it is not as easy to identify carriers in order to eliminate them from a herd and thereby control a disease. In crowded conditions though, such as in feedlots, cases become of great concern. Two examples on one disease with a zoonotic significance that has different characteristics is rabies. It is of great concern when cases are seen in foxes and skunks (or other animals). On the other hand, since bats act as carriers of rabies, these bats are of great concern in areas of the world where rabies carrier bats exist, e.g. South America.
(Note: Read the Rabies articles in JAVMA)
Modes of Escape
A pathogen must escape from a reservoir or source of infection if a disease is to occur.
Modes of Transmission
An infectious agent must find a way to a new host from its source or reservoir. The mode of transmission partly determines whether a disease will be sporadic, epidemic or endemic. It also largely determines the dosage reaching a new host.