1) Necrosis of an essential organ. Either in the brain which does not regenerate e.g. rabies or the virus destroys the organ quicker than it can regenerate e.g. feline panleukopoenia.

2) Immune complex disease e.g. feline infectious peritonitis when immune complexes block the blood vessels of the peritoneal cavity.

3) Immunosuppression. e.g. Feline Immunodeficiency Virus when the virus eventually kills the T helper cells and the cats dies of bacterial infections or tumours.

4) Latency, virus goes into hiding and produces no proteins for the T cells to see, e.g. feline rhinotrachietis herpesvirus in the ganglia. Stress, e.g. of boarding, allows virus production to restart and cause a new bout of sneezing in cats which looked normal on entry.

5) The animal is left with a difficult secondary bacterial infection e.g. sinusitis from feline calicivirus.