ACtually, what Ela typed is an over simplication of things, and it isn't quite that simple. FIP is a mutation of the corona virus - this is something that a large majority of cats will come into contact with during their lifetime and not have any problems with, and even if cats are in the same household as a cat with FIP, they are not guaranteed to get it. There are 4 ways that a cat will react to it and it all depends on their immune system - if they have a very good immune system, they will 'shrug it off', if they have a good immune system they will prob deal with it, but it may stay dormant in their system, and stress in later life could set it off again, a weak immune system will allow the virus to mutate slowly, causing dry FIP, and that is why they can live longer with that form, and a very weak immune system will allow it to mutate rapidly, causing wet FIP. You can also get both wet and dry in the same cat. Good litter tray hygiene is essential if a cat is suspected of it, and Dr Addie has done some tests on cat litters, and some have a lesser risk of it being passed on than others, the chart is on her site.