They are only available through veterinarians, but are safe to feed to other cats in the home of any age. (allfelinehospital.com)
Indoor cats are also more likely to face stressful situations, like fighting with other cats in the home. (petenrichments.com)
FKS, as it is often caused, is more often found with litters of stray and feral cats, so if you are in a kitten foster situation, those kittens should be kept away from other cats in the home until they have all been checked clear of communicable disease. (thesprucepets.com)