Infection can be transmitted both when a cat is going through the acute phase of a disease and is ill or during the incubation period before the cat becomes ill. In addition, recovered cats can be carriers. They are healthy but spread infection.
Horizontal transmission can be by direct cat-to-cat contact or via an inanimate objects or through the air.
In direct contact a part of an animal makes contact with the part of another animal such as when one cat grooms another with his tongue. Fighting is another example. Disease can be transmitted through sexual intercourse in cats. This is not a "significant route of infection in the UK"1.
|Contact via a cat fight. Photo by ilovebutter|
In indirect contact two or more animals come into contact with the same intermediary object. It might be bedding material or a feeding bowl.
While some infectious agents (pathogens) can remain viable in the environment from long-time, particularly in favourable conditions such as dark and damp conditions, they are frequently fragile organisms which are easily killed by disinfectants, drying out, heat and light.
Feline parvovirus can survive for very long periods in the environment. In general, infectious agents that are transmitted indirectly are hardy and more difficult to kill using the usual methods.
We are all aware of the airborne method of spreading disease. The infection is spread in droplets produced when the cat coughs or sneezes. This method of transmission is particularly important with respect to respiratory diseases.
Finally, some infectious agents don't pass directly from one individual to another but they spend part of their life cycle on or in another host with a vector for transmission. The example is the tapeworm affecting cats. Small rodents are used as an intermediate host and fleas as the vector. I have a page on that.
1. The Welfare of Cats K. Sturgess.