The larger the population in a given environment, the more susceptible these cats will be to respiratory disease. Once a cat or kitten has been infected with Calici virus, it will continue to spread it into the environment and to other cats even after recovery. There is no time limit to this, and it can continue for a few short weeks to as long as a few years. All cats must come in contact with the virus to become infected. This virus makes its entrance internally through the eyes, nose or mouth, usually by coming in contact with a carrier or ill cat.
Direct contact is from a sick or carrier cat directly to another cat.
In catteries, multiple cat households and shelters, the chance of this happening are high. As the disease progresses from cat to cat, multiple, repeated infections may occur so that each successive infection produces more virus in each individual. A cat coming in contact with these objects becomes infected indirectly rather than directly from the cat who put it there. People also spread this virus from cat to cat from their hands and clothing. Indirect contact can be a source of infection in shelters, catteries or even veterinary clinics.