Incidence and Cause
Leukemia is seen in animals, such as cats, guinea pigs, and cattle, as well as in humans. In humans it can occur at any age, but most types are more prevalent in older people. Possible causes include exposure to certain chemicals (e.g., benzene), chromosomal abnormalities such as Down syndrome, exposure to ionizing radiation, certain drugs (e.g., alkylating agents used in cancer treatment), and infection with retroviruses such as HTLV-I, a relative of the AIDS virus. All of these agents are suspected of causing mutations or other disruptions that interfere with the normal regulation of cell growth and division in leukocytes.