They Live to be Shared: Parasites
This collection of nasties result in a variety of non-specific clinical signs including lethargy, vomiting, and diarrhea.
Heartworms
Yes, cats can contract heartworms! Even if your cat is not an outdoor cat, it is still susceptible to the bite of a mosquito carrying Dirofilaria immitis. Prevention is much easier than treatment when it comes to heartworm disease. Heartworm medication also controls fleas and intestinal parasites.
Fleas
Fleas are at minimum a nuisance to you and your pet. Cats are often allergic to fleas which can cause extreme pruritus (itching) and hair loss. In more severe infestations fleas cause anemia due to the large amount of blood they suck. Look for "salt-and-pepper" residue on the hair - this is indicative of flea eggs and feces, respectively.
Intestinal Worms
Hookworms, roundworms, whipworms, and tapeworms all potentially infect your feline and even you. Worms are naturally passed on from the queens to kittens via milk and/or in utero or to other cats and humans via the fecal-oral route or breaks in the skin. Annual fecal exams may identify these parasites. Routine deworming with a topical agent is especially prudent for outdoor cats. Intestinal parasites can infect humans and result in GI, skin or eye problems. A monthly topical solution applied between the shoulder blades helps prevent many health issues. Your cat could be a potential source of parasites, therefore keeping cats free of parasites protects the family as well.