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Worldwide, most adults acquire at least one sexually transmitted infection (STI), and many remain at risk for complications. Each day, for example, more than 1 million STIs are acquired worldwide, placing many affected persons at risk for adverse reproductive health outcomes and neoplasia. Certain STIs, such as syphilis, gonorrhea, HIV infection, hepatitis B, and chancroid, often occur in highly interconnected sexual networks characterized by high rates of partner change or multiple concurrent partners. Such networks, for example, often include persons who engage in transactional sex, men who have sex with men (MSM), and persons involved in the use of illicit drugs, particularly methamphetamine. Other STIs are distributed more evenly throughout populations. For example, chlamydial infections, genital human papillomavirus (HPV) infections, and genital herpes can spread efficiently even in relatively low-risk populations. Finally, modern technologies based on detection of nucleic acid have accelerated elucidation of the role of sexual transmission in the spread of some viruses, including Ebola virus and Zika virus, and have provided new evidence of apparent sexual transmission of several bacteria, including group C Neisseria meningitidis and anaerobes associated with bacterial vaginosis (BV).

In general, the product of three factors determines the initial rate of spread of any STI within a population: rate of sexual exposure of susceptible to infectious people, efficiency of transmission per exposure, and duration of infectivity of those infected. Accordingly, efforts to prevent and control STIs aim to decrease the rate of sexual exposure of susceptible to infected persons (e.g., through education and efforts to change sexual behavior norms and through control efforts aimed at reducing the proportion of the population infected); to decrease the duration of infectivity (through early diagnosis and curative or suppressive treatment); and to decrease the efficiency of transmission (through promotion of condom use and safer sexual practices, use of effective vaccines, and male medical circumcision).

In all societies, STIs rank among the most common of all infectious diseases, with at least 40 microorganisms now classified as predominantly sexually transmitted or as frequently sexually transmissible (Table 136-1). In developing countries, with three-quarters of the world’s population and 90% of the world’s STIs, factors such as population growth (especially in adolescent and young-adult age groups), rural-to-urban migration, wars, limited or no provision of reproductive health services for women, and poverty create exceptional vulnerability to disease resulting from unprotected sex. During the 1990s in China, Russia, the other states of the former Soviet Union, and South Africa, internal social structures changed rapidly as borders opened to the West, unleashing enormous new epidemics of HIV infection and other STIs. Despite advances in the provision of highly effective antiretroviral therapy worldwide, HIV remains the leading cause of death in some developing countries, and HPV and hepatitis B virus (HBV) remain important causes of cervical and hepatocellular carcinoma, respectively—two of the most common (and preventable) malignancies in the developing world. Sexually transmitted herpes simplex virus (HSV) infection ...

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