Half of all new human immunodeficiency virus (HIV) infections in the United States and two thirds of all sexually transmitted diseases (STD) occur among young people under the age of 25. It is estimated that by the end of high school, nearly two thirds of American youth are sexually active, and one in five has had four or more sexual partners. Despite these alarming statistics, less than half of all public schools in the United States offer information on how to obtain contraceptives and most schools increasingly teach abstinence-only-until-marriage (or "abstinence-only") education. There is little evidence that abstinence-only programs are successful in encouraging teenagers from delaying sexuality activity until marriage, and consequently, avoiding pregnancy, or STD or HIV infection. Comprehensive sex education, which emphasizes the benefits of abstinence while also teaching about contraception and disease-prevention methods, has been proven to reduce rates of teen pregnancy and STD infection.