This article is from the Feline Infectious Peritonitis FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
Wouldn't we all like to know! Seriously, there seems to be two schools of thought. One group (from Cornell-based publications and seminars) states that the spread is not known with certainty, but is believed to be by ingestion or inhalation of the virus. The other school of thought (from Dr. Pedersen and the UC, Davis based publications) believes that transmission is most prevalent when cats have close contact with other infected cats or their feces/urine. Both schools seem to feel that feces may play a large role in the method of transmission.
Some studies suggest that viruses that can cause FIP can survive on dry surfaces (food/water bowls, litter boxes, human clothing, etc.) and can survive at room temperature probably up to 2 or 3 weeks. If this is the case, then the two schools of thought on methods of transmission may not be so far apart, especially given that litter can contain dust to which small particles of feces can adhere. Thus the virus can possibly be spread via litter dust on shoes or clothing or etc. making it behave as if it were an airborne virus!
If the virus can last so long on dry surfaces, what happens if I unknowingly come in contact with a cat with FIP? Can I give it to my cats?
Most household soaps, detergents and disinfecting agents will kill the virus. Make sure you wash any part thoroughly that has come in contact with the cat (don't forget your pants if the cat rubbed up against you). Bleach in a 1:32 solution is suggested for decontamination purposes.