This article is from the Feline Infectious Peritonitis FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
If it comes in regular contact with other cats (i.e.: an indoor/outdoor cat), the answer is YES! The lowest risk groups are indoor only, single-cat households. The higher the number of cats, the more risk of FIP. The higher the number of cats, the higher the titer test results (more on titers below). Single-cat households are generally free of all coronaviruses. FIP occurs in greatest incidence in cats between six months and two years old, although infections are high up to five years old. Of course the most susceptible group to catching FIPV are kittens because under the age of 16 weeks their immune system is very bad in general.
Studies also show that poor nutrition, high stress levels or poor husbandry increase the likelihood of getting FIP. Outside exposure, exchanging of animals, especially kittens and young cats, highly inbred cats, and cats in actively breeding households increase the risk. Males and females are equally affected.