I lost Festus' sister Mattie to FIP. I actually didn't realize how sick she was, because Festus was bathing her and keeping her clean. Mattie shared bowls and litterboxes with Festus and Garfield, and they were all under 6 months old. Garfield had survived distemper, and has feline herpes, so he is not what one would consider a kitty with a strong immune system.
My vet said FIP is caused by a mutation in the coronovirus, and most cats carry coronovirus. They did not test Festie or Gar, stating that they would test positive for coronovirus, but it would give no indication of whether they would develop FIP. Two years later, they are both going strong.
I believe that all you can do is manage the stress in the kitties lives. High stress can allow FIP to develop. Although fostering is stressful to cats, since I have done it since mine were kittens, I chose to continue. Otherwise, I try to keep a low stress environment for my animals, as much as possible.
Also, FIP is a mutation of coronovirus. So any multicat household with coronavirus can develop FIP. The actual mutated virus is supposedly not spread between cats very easily.
I worried after losing Mattie, especially since Festus became a potbellied girl! Could it be FIP?!?! But she is just a fatty patty...watch your other cats for loss of energy or listlessness. Otherwise, try to relax and realize that most likely Tuffy did not pass FIP to the others.
I am sorry for his loss. Watching Mattie fade away from us was very difficult. She did have a certain peace about her, though, even in her illness. Tuffy is with her, now. She had a very short life, so it will be good for her to hear his stories about the ups and downs of his life. I'm sure he had many adventures!