Get him or her checked for FIV/FeLV. The test is only around 50 dollars and well worth it.
did you know that 23% of cats that are sick test positive for feline leukemia?

If your cat is showing signs of lethargy, behavior changes (ie hiding behind things when they are normally out and about), not eating, lack of grooming, or just sick looking, have this test done. It could spare the lives of your other cats if you have them or else increase the liklihood that treatment will be effective.

You should also consider getting them checked for diabetes or kidney failure if they exibit those signs plus heavy water drinking.

also have their thyroid checked for over active or underactive. and a good clean listen to the heart and lungs.. a good physical check is important for your feline /canine friends.
this i know all to well my baby had feline asthma and an over active thyriod .