The age of the cat at the time of initial FECV exposure may play an important role in whether a cat dies from FIP. Kittens usually began shedding FECV at around 9-10 weeks of age, which places their actual exposure a few days to a week earlier. The immune system of the kitten is rapidly maturing during the period between 6-16 weeks of age. Therefore, the first exposure of most cats to FIP causing mutants occurs during a time period when their immune systems are still developing. This lack of development enhances the likelihood of a FIPV mutant to gain a strong foothold into the body. Just as there is an age susceptibility, there also appears to be an age resistance. FIP is seldom seen in cats over 3-5 years of age, and most cases occur before 16 months of age.