After reading aran's warnings about Psittacosis, I looked it up at the CDC (Centers for Disease Control of the US Government) and found that it is pretty rare in humans (about 50 documented cases per year in the US). See details below.
Another website that looked reliable was
http://www.birdsnways.com/wisdom/ww23eiii.htm
which says: "Most human cases are contracted from psittacines [parrots], pigeons, and turkeys" (NOT CHICKENS
) And it says: "Humans are usually infected by the inhalation of infective particles in the air."
Here are the details from the CDC:
http://www.cdc.gov/ncidod/dbmd/diseaseinfo/psittacosis_t.htm
Psittacosis
Chlamydia psittaci, a bacterium
Clinical Features
In humans, fever, chills, headache, muscle aches, and a dry cough. Pneumonia is often evident on chest x-ray.
Incidence
Since 1996, fewer than 50 confirmed cases were reported in the United States each year. Many more cases may occur that are not correctly diagnosed or reported.
Sequelae
Endocarditis, hepatitis, and neurologic complications may occasionally occur. Severe pneumonia requiring intensive-care support may also occur. Fatal cases have been reported.
Transmission
Infection is acquired by inhaling dried secretions from infected birds. The incubation period is 5 to 19 days. Although all birds are susceptible, pet birds (parrots, parakeets, macaws, and cockatiels) and poultry (turkeys and ducks) are most frequently involved in transmission to humans.
Risk Groups
Bird owners, pet shop employees, and veterinarians. Outbreaks of psittacosis in poultry processing plants have been reported.
My take on it: It seems highly unlikely for a person to contract it from chickens, but if you did, the most likely way would be for an adult to inhale it while cleaning poop, thus aran's advice about wearing a mask while cleaning would be the best prevention, if one is concerned. As for risks to my kids, I'm no longer worried about this one!