Types of cancer, such as leukemia, and the therapies used to treat them (for example, chemotherapy and bone marrow transplants) often decrease the numbers of white blood cells in the body. White blood cells fight bacterial, fungal, and viral infections. Most patients with cancer who have few white blood cells have very weak immune systems and can't fight infections well. They get infections easily and, as a result, can die, particularly if the infections are not recognized early and treated. Fever is usually one of the earliest signs of infection. Thus, doctors prescribe powerful antibiotics whenever they detect a fever and suspect an infection in a patient with cancer whose white blood cell count is low. Doctors usually give combinations of antibiotics that work against many different types of bacteria. We do not know whether certain combinations work better and cause fewer side effects than others.