In certain cats, eg. those who are genetically predisposed to this, exposure to coronavirus can develop into FIP. Coronaviruses/FIP constantly mutate, which makes testing for it difficult at best. It does begin as a GI infection/virus, but that does not mean that in your cat (or every cat) it will become FIP.

Our kitten who had FIP tested negative for FIP on the ELISA test just a week before we had to put her to sleep. We were told that the antibodies can become bound up in the virus, leaving few-none to "attach" to the test.

If your cat is eating and growing normally, and is otherwise healthy (besides the diarrhea); no fevers, no inaapetance, no unusual lethargy, then it does not sound like it's FIP (although our resident vets, like Meghane would know better).

It is very complicated. Perhaps Meghane can offer a simple explaination. But I can offer you this experience:
Many years ago (1987), a sweet, starving, flea infested street kitten came into my life. I called her Princess. She was about 9 weeks old and weighed Princess was eating, drinking and even playing. I remember being very frustrated (like you)with putting all of this money into tests and meds that didn't seem to be going any where. Eventually, the vet said something like this: He described something called a Corona Virus...like all viral infections, it is not curable, but either the host will build up immunity, or will eventually weaken and die from it. He said that most kittens get the necessary immunity factors from their mother's milk, but in cases where the mother might have been killed, or abandoned the kitten, this would not be the case. His guess was that Princess would survive, because he had put on a little weight and was eating and active. He said that the virus would never go away completely, but a benign version would retreat into the cells where it would be supressed by her immune system. When I asked about a definitive test, he said that that would be expensive...and why bother of it wouldnt effect the outcome?

Princess' loose BMs did go away soon after, and she lived to be 13 years old. She was a very healthy cat until all of a sudden she started to lose a drastic amount of weight. To make a long story short, my vet (a different one from the guy who diagnosed her as a kitten) and I decided to put her down: A thousand dollars worth of tests (bloods and sonograms) showed some kind of liver enzyme problem. When I told this vet about the possibility that she had Corona as a kitten, I remember her remarking that it might have morphed into something like FIP, but that's all I really remember.

Lisa

If this condition is/can be so serious why would the vet barely speak of it and brush it off and tell me it was nothing to worry about and that "almost all cats have this in their systems". The vet barely mentioned this. She didn't even offer to tell me the name of the bacteria (she said that this was found when they tested for bacteria) Is it contagious to my other cats?

It is my understanding that most (all?) cats are/have been exposed to cornovirus at some point in their lives. Supposedly the percentage of cats in whom it mutates into FIP is low (or at least relatively so). This could be why it was not mentioned. Most cats are exposed to the cornovirus, and are perhaps sick for a few days, and that's the end of it. There are a lot of other diarrhea-causing bacteria and parasites (Giardia, Coccidia, worms, etc.) affecting many rescue kittens/cats as well. Almost all cats will test positive for coronavirus antibodies (indicating prior exposure) and knowing that really doesn't tell you much. There is no accurate test for FIP (most diagnoses are anectodal, the only 100% accurate tests are done through necropsy; biopsy of liver for example after the cat/kitten has died), nor is there a truly reliable prevention, and there is no cure. I think vets sometimes deliberately refrain from bringing it up in order to avoid unnecessarily worrying their clients. Having volunteered at an animal shelter for the past few years, I myself have seen a number of kittens who I thought could have had FIP, purely based on my own FIP kitten's symptoms/experience with it, and it turned out that they had had other cause(s) for their symptoms, got well, and went on to grow up and live their lives.

Yes, coronavirus is highly contagious, but your other cats might never become ill or might have already experienced some very brief and mild symptoms, so it passed by unnoticed. They might shed the virus for a short time afterward, or might never shed the virus. There is too much unpredictability with this virus to be sure.

One of our cats we'd gotten as a kitten a couple months after bringing home our first kitten (with FIP). While it is entirely possible that she could develop FIP much later in her life (from what I've read and heard, the disease tends to develop in cats with compromised or weakened immune systems, generally young kittens, and senior cats), at the moment she is a (chubby) 5 year old cat, happy and healthy. We brought home another kitten about 4-4 1/2 months after losing our sick kitten. She's lived with us since she was ~7 weeks old, and the now 5 yr old cat was 9 1/2 months. So far, so good....