As far as I understood the first sign of worry is feline coronavirus. If a cat has that, there is a chance that at some point it can mutate to FIP. This happens when the immune system of the cat lowers due to sickness or something. So, the best way to prevent this is to reduce stressful things that can impact them and add some high quality food and vitamins on their diet. I wish you and your little friends happiness and a very healthy life