feline immunodeficiency virus (fiv)

This virus is mainly transmitted through cats biting each other or a mother may pass the infection to her kittens. Affected cats have a poorly functioning immune system and are vulnerable to developing a variety of secondary infections such as mouth infections, chest infections and gut problems. Weight loss and loss of appetite may also be seen. Mild disease that comes and goes may be seen in the early stages but this becomes more persistent and severe over time.

SOURCE: cats.org.uk