The first AIDS cases—otherwise healthy young men with multiple infections and cancers—were a mystery to even the most seasoned physicians. The symptoms pointed to a major defect in the immune system. Further investigation found swollen lymph nodes, another sign of immune stress. A clear hypothesis emerged: the cells of the immune system were directly infected. Tissue cultured from patients' lymph nodes revealed a new virus—a retrovirus. This type of virus contains RNA that it converts to DNA once it infects human cells. Named human immunodeficiency virus, or HIV, its viral code integrates into the host genome, a safe haven from most drugs, and causes lifelong infection. HIV infects lymphocytes throughout the body, disabling the very cells that defend against invading viruses. With a weakened immune system, patients are vulnerable to infections that are normally easy to fend off. A detailed understanding of the HIV life cycle, from cell attachment and entry to translation and assembly of new viruses, can help researchers identify targets for drug therapy.