You say fat, which makes em thing bloated - which implies Dropsy, which is very bad.
Dropsy is a bacterial infection caused by poor water quality. The fish appears bloated, and in later stages it's scales will stick out, giving it a pine cone like appearance - at this stage is is basically terminal. Earlier signs include lethargy and lack of appetite.
It is not contagious, luckily. But because it is caused by poor water quality is is possible that other fish will get it as well because they all could be suffering.
I'm not saying this is what it is, to be honest I need more information to be sure.
Could you give some more information? Whats the size of the tank, and what other fish do you have in the tank? Do you have a testing kit that can determine what the Ammonia, Nitrate, Nitrate and pH of the tank are? Could post a picture so we can get an idea of exactly what is wrong with him?