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: . 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 . 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. Infection of macrophages by FIPV is responsible for development of a fatal granulomatous vasculitis, or FIP (see granuloma). Development of FIP depends on two factors: virus mutation and low immunity where virus mutation depends on the rate of mutation of FECV to FIPV and the immune status depends on the age, the genetic pool and the stress level. High immune status will be more effective at slowing down the virus. Molecular biology Two forms of feline coronavirus are found in nature: enteric (FECV) and FIP (FIPV). There are also two different serotypes found with different antigens that produce unique antibodies. FCoV serotype I (also called type I) is the most frequent. Type I, that can be defined as 'FECV that could mutate to FIPV type I', is responsible for 80% of the infections. Typically, serotype I FCoV cultures are difficult to perform, with few resulting studies. FCoV serotype II (also called type II) is less frequent and is described as 'FECV type II that can mutate to FIPV type II.' FCoV type II is a recombinant virus type I with spike genes (S protein) replacement from FCoV by the canine coronavirus (CCoV) spikes. The type II cultures are generally easier to perform, which has resulted in an imbalance of experiments performed with many studies about type II (even though it is a far less common form). More recent research points to a common ancestor between FCoV and CCoV. This ancestor gradually evolved into FCoV I. An