Domestic cats are usually harmless animals that can occasionally cause serious infections by bacteria, parasites, fungi, and viruses through bites, scratches, contact with their feces, or flies. Their narrow, sharp teeth can deposit pathogens deep into the tissues of those whom they have bitten, allowing a seemingly minor wound to progress to a fulminating infection.

Such can be the case with Pasteurella multocida, a bacterium first found in 1878 in cholera-infected birds, isolated by Louis Pasteur in 1880 and named in his honor. Normally, this Pasteurella lives harmlessly in the mouths of healthy cats, but occasionally it can be introduced deep into tissues and cause severe local damage or a life-threatening infection, especially in joints or bones.

Another rare and severe bacterial infection can be caused by Capnocytophaga bacteria, normally carried in saliva or dental plaque. More common and far less dangerous is “cat-scratch disease,” caused by Bartonella henselae and usually acquired from flea bites and flea feces. It causes a small blister or pustule where the scratch or bite has occurred, sometimes followed by swollen lymph nodes and flu-like symptoms. In severe cases, treatment with penicillin or draining with a needle may be required, but most cases get better on their own.

Cats can carry intestinal bacteria such as Salmonella and Campylobacter, sometimes without showing symptoms. Humans can contract these bacteria through contact with feces, contaminated surfaces, or contaminated pet food, especially from cats that hunt or eat raw meat. Other rarer but notable bacterial diseases are plague (Yersinia pestis) and tularemia (Francisella tularensis). In parts of the western United States and other regions where these bacteria circulate in wild rodent populations, outdoor cats can become infected and then transmit the disease to people through bites, scratches, or respiratory droplets.

Cats can also be the definitive host for Toxoplasma gondii, a microscopic, single-celled parasite that can cause toxoplasmosis in humans. Interestingly, this organism can complete its reproductive cycle only in cats. Although a huge percentage of cats are exposed to toxoplasmosis during their lives, most never get sick or show symptoms because their immune systems efficiently control the parasite. As a result, the disease can affect only young kittens or cats with weak immune systems. However, for just a few weeks after first becoming infected, an infected cat sheds microscopic oocysts in its feces, and humans can be exposed by accidentally ingesting these oocysts, for example through contaminated soil, gardening without gloves, or cleaning a litter box and then touching the mouth.

Cats can also carry intestinal parasites such as the roundworm Toxocara cati and hookworms. Eggs and larvae pass in the feces and contaminate soil and sand, where children playing outdoors are most likely to encounter them. When humans ingest roundworm eggs, the larvae can migrate through the body, a condition called visceral or ocular larva migrans, which in rare cases damages the liver, lungs, or eyes. Hookworm larvae can burrow through the skin, causing an itchy, winding rash known as cutaneous larva migrans. Giardia and Cryptosporidium are microscopic protozoa that can infect cats and cause diarrhea, and may occasionally spread to people through fecal contamination.

Ringworm, despite its name, is not a worm but a fungal infection, most often caused by Microsporum canis. It is highly contagious and spreads through direct contact with an infected animal or with contaminated bedding, brushes, or furniture. In humans, it produces circular, red, scaly, itchy patches on the skin or scalp. Kittens and long-haired cats are common carriers. Some adult cats carry the fungus without visible signs. Cats can also carry external parasites that directly affect humans. Fleas bite people and can transmit Bartonella and tapeworms, while scabies mites can cause itchy skin rashes. Ticks carried by outdoor cats may bring their own illnesses into the home.

Rabies is rare in vaccinated pets but remains the most feared zoonosis because it is almost invariably fatal once symptoms appear. In the United States, cats are reported with rabies more often than dogs, largely because vaccination rates among cats are lower and outdoor cats may encounter infected wildlife such as bats, raccoons, and skunks. Rabies is transmitted through saliva, usually via a bite. Therefore, vaccinate cats, keep them indoors when possible, and seek immediate medical care for bites. Cats can also be infected by certain respiratory pathogens, and during the COVID-19 pandemic, some domestic cats caught SARS-CoV-2 from infected owners. Evidence of cats passing it back to humans is very limited, but the episode highlighted that animal and human health are interconnected.

The cat seems to have become one of man’s best companions, at least since, according to Rudyard Kipling’s famous short story “The Cat that Walked by Himself,” it got tired of walking alone. The experience of cat owners would indicate that every cat seems to have a somewhat different personality, some exceedingly pleasant and others quite mean. With the decline of large families, cats increasingly serve as a remedy for the loneliness of old age. Technically, cats without a home or family are called feral. Many decades ago, so many feral cats were in our hospital parking lot that an administrator issued a stern written warning to the staff against feeding them. Unfortunately, his secretary misspelled feral as fecal, a source of considerable amusement to our anti-administration views and attitudes of that time and age.