Benefits In humans, sex has been claimed to produce health benefits as varied as improved sense of smell, stress and blood pressure reduction, increased immunity, and decreased risk of prostate cancer. Sexual intimacy, as well as orgasms, increases levels of the hormone oxytocin, also known as "the love hormone", which helps people bond and build trust. A long-term study of 3,500 people between 30 and 101 by clinical neuropsychologist David Weeks, MD, head of old age psychology at the Royal Edinburgh Hospital in Scotland, found that "sex helps you look between four and seven years younger", according to impartial ratings of the subjects' photos. Exclusive causation, however, is unclear, and the benefits may be indirectly related to sex and directly related to significant reductions in stress, greater contentment, and better sleep that sex promotes. In contrast to its benefits, sexual intercourse can also be a disease vector. There are 19 million new cases of sexually transmitted diseases (STD) every year in the U.S., and worldwide there are over 340 million STDs a year. More than half of all STDs occur in adolescents and young adults aged 15–24 years. At least one in four U.S. teenage girls has a sexually transmitted disease. In the US, about 30% of 15–17 year old adolescents have had sexual intercourse, but only about 80% of 15–19 year old adolescents report using condoms for their first sexual intercourse. More than 75% of young women age 18–25 years felt they were at low risk of acquiring an STD in one study. Chlamydia is particularly dangerous because there are many infected individuals who experience no symptoms. Left untreated, chlamydia can lead to many complications, especially for women (such as the inability to bear children later in life). Hepatitis B can also be transmitted through sexual contact. The disease is most common in China and other parts of Asia where 8–10% of the adult population is infected with Hepatitis. About a third of the world's population, more than 2 billion people, have been infected with the hepatitis B virus. Syphilis infection are on the rise in all parts of the United States and is related to about 21% of fetal and newborn deaths in sub-Saharan Africa. Syphilis causes genital sores that make it easier to transmit and contract HIV. AIDS is caused by HIV which is spread primarily via sexual intercourse. The World Health Organization reported that in 2008, approximately 33.4 million people had HIV (about 2/3 in sub-Saharan Africa and 1.1 million in the United States ) , and 2 million died of AIDS worldwide. Safe sex is a relevant harm reduction philosophy. Condoms and dental dams are widely recommended for the prevention of STDs. According to reports by the National Institutes of Health (NIH) and World Health Organization (WHO), correct and consistent use of latex condoms reduces the risk of HIV/AIDS transmission by approximately 85%–99% relative to risk when unprotected.