Sexually transmitted infections (STIs) are a set of infectious diseases caused by different microorganisms. They all have in common that they are spread, most of the time, through sexual intercourse. More than 20 STIs are known, including chlamydia and lymphogranuloma, gonorrhoea, genital herpes, human immunodeficiency virus (HIV), genital warts caused by human papillomavirus, syphilis and trichomoniasis. Although most STDs affect both men and women, women have the most serious health problems. Not all STDs can be cured: those caused by bacteria, such as chlamydia, are treated with antibiotics, but in those that are caused by viruses, such as genital herpes, treatment is limited to alleviating the symptoms.