Young cats (younger than two years), older cats (older than ten years), cats in poor physical condition, and cats undergoing infection or stress (in short: high stress and/or low resistance) are more susceptible to FIP. It is a relatively rare disease in the general cat population; less than 1% of cats seen by a veterinarian. In multi-cat households, such as shelters and catteries, the percentage may be higher, sometimes 10-20% of the susceptible population over several months.