Feline coronavirus (FCoV) is a positive-stranded RNA virus that infects cats worldwide. It is a coronavirus of the species Alphacoronavirus suis, which includes canine coronavirus (CCoV) and porcine transmissible gastroenteritis coronavirus (TGEV). FCoV has two different forms: feline enteric coronavirus (FECV), which infects the intestines, and feline infectious peritonitis virus (FIPV), which causes the disease feline infectious peritonitis (FIP).

Feline coronavirus is typically shed in feces by healthy cats, and transmitted by the fecal-oral route to other cats. In environments with multiple cats, the transmission rate is much higher compared to single-cat environments. The virus is insignificant until mutations cause it to be transformed from FECV to FIPV. FIPV causes feline infectious peritonitis, for which treatment is generally symptomatic and palliative only. The drug GS-441524 shows promise as an antiviral treatment for FIP, but at the moment it still requires further research. The drug GC376 is also being studied and developed.

Feline coronavirus is found in cat populations around the world. The only known exceptions are on the Falkland Islands and the Galapagos, where studies found no occurrences of FCoV antibodies in cats tested.

Feline enteric coronavirus is responsible for an infection of the mature gastrointestinal epithelial cells (see also enterocytes, brush border, microvilli, villi). This intestinal infection has few outward signs, and is usually chronic. The virus is excreted in the feces of the healthy carrier, and can be detected by polymerase chain reaction (PCR) of feces or by PCR testing of rectal samples.

Cats living in groups can infect each other with different strains of the virus during visits to a communal litter tray. Some cats are resistant to the virus and can avoid infection or even becoming carriers, while others may become FECV carriers.

The virus becomes feline infectious peritonitis virus (FIPV) when random errors occur in the virus infecting an enterocyte, causing the virus to mutate from FECV to FIPV.

In their pre-domestication natural state, cats are solitary animals and do not share space (hunting areas, rest areas, defecation sites, etc.). Domestic cats living in a group therefore have a much higher epidemiological risk of mutation. After this mutation, the FCoV acquires a tropism for macrophages, while losing intestinal tropism.

In a large group of cats, n, the epidemiological risk of mutation (E) is higher and expressed theoretically as: E = n² −n. A house hosting 2 cats therefore has risk of mutation E = 2. When 4 kittens (6 cats in total) are born into this house, the risk increases from 2 to 30 (6²−6). Overcrowding increases the risk of mutation and conversion from FECV to FIPV, which constitutes a major risk factor for the development of feline infectious peritonitis (FIP) cases. FIP has been shown to develop in cats whose immunity is low; such as younger kittens, old cats, immunosuppression due to viral—FIV (feline immunodeficiency virus) and/or FeLV (feline leukemia virus) and stress, including the stress of separation and adoption.