There are a lot of illnesses that affect cats. One of them is called feline infectious peritonitis. 

Feline Infectious Peritonitis

FIP is one of a group of viruses called coronavirus. In cats, coronavirus attacks the cells of their intestinal walls. The particular virus that causes FIP has been isolated, as has one that is closely related to feline infectious peritonitis but is generally much less severe. 

Symptoms

There are two different kinds of FIP, wet and dry. They have some symptoms in common. One of those is a fever that doesn't respond to any antibiotics that your vet prescribes. Another is lethargy and weight loss. Wet FIP has additional symptoms. One of those is fluid buildup in the lungs or belly. If your cat has fluid buildup in their lungs, then that can cause them to have a hard time breathing. If the fluid is building up in their belly, then your cat's belly will slowly start to expand. Generally, their bellies won't be painful. 

In the dry form, one of the specific symptoms is inflammatory cells on the affected organs. However, that's not something that's easily visible to you, but the results of those cells are. If your cat's kidneys are involved, then you are going to see things like excessive thirst which will also cause excessive urination. Your cat is also going to be vomiting a lot. If the liver is affected, then you will see some jaundice in your cat. Since you can't see a lot of your cat's skin, it can be hard to tell if their skin is yellowing. You will have to see it in their eyes. 

Diagnosis

Sadly, there isn't one definitive test that your vet can do that will say if your cat has FIP or not. Your vet can do a pet ultrasound to see if they can figure out why there is fluid building up in your cat's belly or lungs, as well as draining off some of the fluid and analyzing it. They can also do a biopsy of affected organs, but that will mean some kind of surgery. There is a blood test that your vet can do that will show whether or not they have a coronavirus in their system, but it won't be able to differentiate between FIP and its closely related, but less severe, cousin. 

If you think that your cat has FIP, you should immediately take them to the vet so they can be checked out.