A healthy adult person has little to fear from keeping chickens, but those with impaired immune systems due to age, cancer, HIV infection, diabetes, or other medical conditions should resist the impulse to bring home cute, fluffy chicks from the feed store. Raising chickens may not be a wise pursuit for them.
Several chicken-borne diseases are unpleasant for healthy adults, but they can cause severe illness and death in people with weak immune systems. High-risk diseases for immunosuppressed people include
Campylobacteriosis. This is one of the most common bacterial infections of humans. It causes cramps, pain, fever, and severe diarrhea. Campylobacteriosis is often a foodborne illness, but contact with pets (especially puppies), poultry, and livestock can also cause it. Campylobacter is the most common cause of "traveler's diarrhea" and usually goes away on its own. In a person with a weakened immune system, the bacteria can cause serious complications.
Salmonellosis. A salmonella infection causes diarrhea, fever, vomiting, and abdominal cramps. The illness lasts up to seven days and most people recover without treatment. But some cases of the diarrhea can be so severe that a person becomes dangerously dehydrated and must be hospitalized. Humans can pick up salmonella from chickens by handling infected eggs. Undercooked meat, unwashed fruits and vegetables, and reptiles can also cause salmonella infection.
Avian tuberculosis. Most humans are highly resistant to this bacteria, but people who have AIDS, who are undergoing cancer treatment, or who have had an organ transplant are susceptible to it (and one strain of avian tuberculosis is resistant to antibiotics). Avian tuberculosis in humans causes localized wounds on the skin with swelling of lymph nodes on the same parts of the body.
Parrot fever. In humans, symptoms of parrot fever range from nothing to high fever, cough, and severe pneumonia. Rose-colored spots may appear on the skin, and in some cases a patient becomes unresponsive or comatose. As bad as it sounds, parrot fever has only a 1% fatality rate in humans. People can pick up parrot fever from chickens, pigeons, macaws, ducks, gulls, and many other bird species.
Histoplasmosis. Going by many names (Cave disease, Ohio valley disease, Darling's disease), histoplasmosis starts with a cough and flu-like symptoms but can advance to resemble tuberculosis. While it often affects the lungs, it can spread to other organs and be fatal if not treated. Contact with contaminated bird and bat droppings are the most common causes of bird-to-human infection.
To be fair, pet chickens aren't known to be the most common sources of infection for any of these diseases. (Dogs and cats can carry Campylobacter and Salmonella, too, and household birds are more likely than chickens to carry the bacteria that cause parrot fever.) But because the risk from chickens is small but real, the U.S. Centers for Disease Control and Prevention and many physicians discourage people with weak immune systems from keeping pet poultry. If you're concerned, talk with your doctor.