The greatest risk factor for bird flu seems to be contact with sick birds or with surfaces contaminated by their feathers, saliva or droppings. In very few instances, bird flu has been transmitted from one human to another. But unless the virus begins to spread more easily among people, infected birds present the greatest hazard.

The pattern of human transmission remains mysterious. People of all ages have contracted bird flu, though the average age of people affected seems to depend on the type of bird flu. The average age of those affected by H7N9 was 62, while the average age of those with H5N1 was only 26.

Recent research suggests that females may be at higher risk of bird flu and its complications. However, it's not clear exactly why that might be so.

Sept. 25, 2014