Tremendous strides have been made in the treatment and outlook for patients infected with HIV, the human immunodeficiency virus. Treatment with a combination of antiretroviral drugs can keep patients with HIV alive for decades, without symptoms of the infection. The trouble is, if HIV-infected people stop taking their medications, the virus takes over in full force again—because the virus hides out quietly in cells of the immune system, kept in check, but not killed by the treatment.