This chapter reviews some basic information about human immunodeficiency virus (HIV) and acquired immune deficiency syndrome (AIDS). Once in the body, HIV affects specific cells, called CD4 cells or T cells, which are part of the body's immune system. It is also important to note that HIV can mutate into forms that are drug resistant which may make certain medications ineffective in stopping its progression. Sexually transmitted infections (STIs) are most often classified by what causes them, in part because that helps explain whether they can be cured. Human papillomavirus (HPV) is the most common STI in the United States. Many public health experts regarded the HPV vaccines as huge medical breakthroughs capable of saving millions of lives. Because HPV is so easily spread through sexual contact, however, they explained that the only way to eliminate HPV forever would be to vaccinate everyone or nearly everyone.