**I am NOT a veterinary professional, this is all my own opinion based on my own experience and research. I will include further resources below**
Over the years, we've lost several kitties and been made aware of several adopted kitties who have passed away (likely) from something called FIP. Feline Infectious Peritonitis, or FIP can happen when a virus that almost all kitties (especially in a rescue/shelter setting) are exposed to, mutates in a largely unknown way, and they are not able to fight it off.
There are different types of FIP and it cannot currently be accurately diagnosed. However, there are warning signs, and tests that can be run that give 'clues' that kitties symptoms may be stemming from FIP, but no clear cut test. In the past, if a kitty was suspected of having FIP, typically they were humanely euthanized... FIP was a death sentence.
BUT, there is some good news. In February 2019, a study was published by UC Davis (General Feline Infectious Peritonitis Resources) regarding the efficacy of a new drug against FIP. This was a BIG deal, and the study showed GREAT results - showing that FIP may in fact be treatable. 25 of the 31 cats studied were successfully treated.
However, this drug has NOT approved or made widely available for use... yet! But it has been shown over and over again to be effective in kitties 'diagnosed' with FIP.
It is also extremely expensive. Approximately 1 week's worth of injections is almost $100. With at least 12 weeks of injections and regular vet visits and blood work and other testing, it's not an easy process... but it is a HUGE leap for FIP kitties - FIP may no longer be the death sentence it has been for so long.
In 2020, we noticed one of our foster kittens, Stitch, was acting funny, not gaining weight like he should, but had a BIG BIG belly. It was FULL of fluid, a telltale sign of FIP. We worked with our vets to do bloodwork and diagnostic testing and he was 'diagnosed' with FIP. Typically at this point, kitties are not eating, have no energy, are lethargic, and you can tell they don't feel good. Typically they don't have long.