Medical reasons

The body hair of surgical patients may be removed before surgery. In the past this may have been achieved by shaving, but that is now considered counter-productive, so clippers or chemical depilatories may be used instead.^[8] The shaving of hair has sometimes been used in attempts to eradicate lice or to minimize body odor due to accumulation of odor-causing micro-organisms in hair. Some people with trichiasis find it medically necessary to remove ingrown eyelashes. Incorrect shaving (shaving against the grain) can often cause ingrown hairs.

Many forms of cancer require chemotherapy, which often causes severe and irregular hair loss. For this reason, it is common for cancer patients to shave their heads even before starting chemotherapy.

In extreme situations people may need to remove all body hair to prevent or combat infestation by lice, fleas and other parasites. Such a practice was used, for example, in Ancient Egypt.

http://en.wikipedia.org/wiki/Hair_removal#Medical_reasons