Immune deficiency, which in fact means an inadequate immune response, may occur for several reasons. For example, it is a side effect of most chemotherapy drugs used to treat cancer. Immune deficiency also occurs naturally. Although the immune system is not functional at birth, infants are protected by antibodies they have received from their mother through the placenta, and infants who breastfeed also receive antibodies from their mother’s milk. These antibodies offer protection until the infant’s own immune system develops during the first months of life. In rare cases, the immune system fails to develop, leaving the child without immune protection. Exposure to any virus or bacterium can be fatal to these children. Therefore, they are sealed into sterile quarters to isolate them from the microorganisms that are part of the normal world.