(taken from [link])

Overview
Feline infectious peritonitis (FIP) is a serious, nearly always fatal systemic viral disease of wild and domestic cats caused by a coronavirus, a type of virus that afflicts pigs, dogs, and in some forms, humans. However, the specific virus that causes FIP — the FIP virus — does not infect humans. The clinical name for the virus that causes FIP is feline coronavirus. (FcoV).

A common, relatively benign form of feline coronavirus is the feline enteric coronavirus (FECV). When FECV mutates into a disease-causing form, it is called FIPV (feline infectious peritonitis virus). If a cat’s immune system responds poorly, the mutant FECV may cause the systemic infection called FIP.

The onset of the disease may be sudden, especially in kittens. It often develops so gradually that the infection is well established before overt symptoms appear. When symptoms become apparent, they often include small tissue abscesses (pyogranulomatous lesions) in the liver, kidneys, and the membrane that lines the abdomen (peritoneum); and fluid accumulation within body cavities.

Types

FIP manifests in two forms: effusive, or “wet,” and noneffusive, or “dry.” The wet form comprises 60% to 70% of all cases and has a more rapid progression than dry FIP.

In wet FIP, fluid accumulates in body cavities — typically in the abdominal cavity, where it causes progressive, but generally painless, swelling; or in the thoracic (chest) cavity, where it causes respiratory distress due to compression of the lungs and fluid backing up into the airways.

In dry FIP, fluid buildup is minimal and symptoms depend on which organs are affected by lesions. About half of all dry cases produce eye inflammation or neurological problems: paralysis, unsteady gait, and seizures. Other lesions may cause kidney or liver failure. Weight loss, pancreatic disease, depression, anemia, and fever are usually present.

Despite these differences, wet and dry FIP are not two different diseases. The strength of a cat's immune system response appears to determine which form of the disease it develops. If the immune system reacts poorly, the cat probably will develop the wet form. A stronger response might result in the dry form. When the cat’s immune system responds optimally, the animal develops neither form but may become a virus carrier for several years. A carrier cat whose immune system weakens over time eventually may develop the full-blown disease.

Incidence and Prevalence
There are two strains of FIPV. The Type I strain, or FCoV-1, is believed to cause about 85% of all identified FIP cases. The less common Type II strain, or FCoV-2, accounts for the rest. FIPV probably affects less than 1% of the cats brought to veterinarians for treatment.

Coronavirus infections are common in cats, especially those in catteries and shelters where large numbers of kittens and adult cats share living space. It has been estimated that 80% to 90% of all the animals in multi-cat households where FECV is present become infected.

The prevalence of FIPV, on the other hand, remains low in the wild and domestic cat populations, probably less than 2%. In multi-cat households or catteries where the disease is present, it can be as high as 10%. In some shelters and catteries, the disease rate can be as high as 20% over a period of several months.
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i'm going to go and cry now. goodnight.