It is more risky for cats to be infected by Feline Infectious Peritonitis (FIP) in multi-cat families. FIP is a deadly infectious disease caused by a virus which is mutated by the Feline Enteric Coronavirus in the cat’s intestines. Why is it more easily to have this disease in multi-cat families? It is because the virus could be spread through excretions such as mouth, respiratory secretions and excrement. Furthermore, the infected cats would be prone to other immunosuppressive diseases, such as Feline Leukemia Virus (FeLV) infection and Feline AIDS (FIV), etc.