If you suspect your cat is sick, get a diagnosis as soon as possible. A vet will be able to tell you what's causing your cat to be sick, what medications he needs to take and any special care or instructions you have to follow to ensure a quick recovery.
Make Him Comfortable
A sick cat doesn't want to be bothered by you or other household pets. The ailing cat needs a quiet, warm place to rest and recover. Locking the cat in the bathroom or confining him to one single room is a good idea, but make sure the place is warm. You might need to provide a hot water bottle or turn up the heat in the room to make the place more comfortable. You will also need to move the litter box into the same room, and place it near the area where your cat is sleeping, so he doesn't have to move long distances to use it.
Check Food and Water Intake
Keep an eye on how well your cat is eating and drinking. If he refuses to eat, try warming up wet food to increase its smell and make it more attractive.
Add chicken, tuna or fish oil to his regular food to make it more appetizing. Hand-feeding might work better for some sick cats. If your cat is not drinking, try administering water using a syringe.
Keep Up With Care
Make sure you give your cat any prescribed medications as directed by your vet for the duration of the recommended course of treatment. Some cats are resistant to taking pills. If that's the case, talk to your vet about the possibility of crushing the pills or switching to a liquid medication.