Introduction
Feline infectious peritonitis (FIP) is an important disease to the cat clinician for several reasons; it is fatal in most (clinical) cases, its biology is poorly understood and prevention is difficult, to say the least. It is also an enigmatic disease: a sporadic viral condition is a contradiction in terms. One expects epidemic expansion from virus diseases, or at least a consistent pattern of spread in a cat society. Another mystifying trait is the fact that antibodies - the molecules we associate with immunity and protection - have no beneficial effect for the cat. Indeed, under certain circumstances they may even precipitate disease, causing the 'early death' phenomenon. The detection of antibodies is of no benefit to the consulting veterinarian either, since titres are meaningless for the purposes of diagnosis and prognosis in individual patients. Nevertheless, there is a place for serology, as we will discuss later. There is also a vaccine available in some countries, which has been shown to provide some protection [4]. However, its efficacy is a matter of debate.

Feline coronaviral polyserositis, as it should be termed, is the fatal 'tip of the iceberg' to a common feline infection with a group of ubiquitous viruses. Most of these coronaviruses are harmless and perfectly adapted to growth in the gut. They have been named 'feline enteric coronaviruses', to distinguish them from the killer viruses that replicate in the feline macrophage. Persistently infected, healthy cats play the most important epidemiologic role in FIP, because by harbouring feline coronaviruses (FCoVs) in their intestines and blood, they act as a constant source of infection. The virus is shed in the faeces, saliva and perhaps other body fluids of infected cats. In addition to these 'pathotypes', coronaviruses also occur in two serotypes, both of which can cause FIP after having undergone subtle genetic changes. It is only for reasons of convenience that we shall continue to use the term FIP virus (FIPV) - to denominate those FCoV strains that carry the mutation(s) responsible for the increase in virulence. The legitimacy of such a nomenclature is questionable, however; it is like giving different names to a virus and its attenuated vaccine strain.

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