The effusive (wet) form results in a build up of fluid in the abdomen (ascites) or in around 10% of cases the chest (pleurisy). This leads to swelling of the abdomen or dyspnoea (difficulty breathing). On set of the disease may be variable with vague signs of pyrexia (fever), diarrhea, weight loss, and jaundice (yellow gums ,eyes, etc.) before the main symptoms are recognized.

The non-effusive (dry) form has the formation of pyogranulomatous (inflammatory tissue) lesions which have the appearance of tumors in a number of organs throughout the body. Kidneys and other abdominal organs, eyes and blood vessels associated with the brain are more common sites. The symptoms relate to the location of the lesions. Neurological (nerve related) symptoms range from seizures to incoordination and paralysis. Generalized early signs are similar to the wet form.

One of the most important factors in the development of clinical disease following infection with FIP is the reaction of the cat's own immune system. The virus enters the body through the mouth and after multiplying within lymph nodes (glands) around the intestine, spreads to the blood (viraemia). Its here that the body's immune system mounts a reaction to eliminate the virus, however in some cats there is an over reaction (similar to an allergy) forming immune complexes which damage the blood vessels. The result is either leaking vessels causing the wet form of FIP , or microscopic abscess within the vessel wall which eventually develop into the dry pyogranulomatous lesions. This has implications in the epidemiology (how the virus is spread). FIP is more common where larger groups of cats are housed together. The coronavirus does not survive for long in the environment so most infection is directly from other infected cats. Most cats within a household will become infected shortly after the virus is introduced however only a small number will develop immune complexes and clinical disease. It is thought stress, other viruses such as Feline Leukemia Virus, breed predisposition (particularly Burmese), and previous exposure to FIP may all contribute to this outcome. It is thought that prior exposure to the virus may prime the immune system to over react when infected at a later stage (similar to common bee sting type allergies).

Diagnosis of FIP is difficult. Blood tests to detect coronavirus antibodies are often open to interpretation. Fluid from wet FIP is usually thick, yellow , and will clot when left to stand. It generally has low numbers of cells and high globulin (type of protein). This increased globulin is also seen in the blood and on serum electrophoresis. High neutrophils and low lymphocytes (types of white blood cells) may also be observed.

There is no effective treatment for FIP.