The Centers for Disease Control and Prevention (CDC) in Atlanta estimate that about 12 million U.S. residents develop an STD each year, and that at least 50 percent of all people in the U.S. will develop an STD at some time before they are 35 years old. The STD that a person is likely to develop has to do with where they live as well as with whom they engage in risky sexual behavior. It also has to do with the characteristics of the organism causing the STD.
Many factors contribute to the development and maintenance of STDs throughout the world - not just individual sexual behavior. There are many reasons why certain organisms are concentrated in certain geographic regions, and why certain sub- populations are at higher risk of getting an STD than others. Rural-to-urban migration, poverty, war, and lack of access to medical care or medicines are reasons why diseases explode within certain populations. While these issues exist worldwide, often they are more intense in tropical developing countries where there are fewer resources.
Certain age groups are more vulnerable to acquiring STDs. Adolescents and young adults are at the highest risk. This is, in part, because adolescents and young adults are more willing to engage in risky activities and are most resistant to adopting prevention methods. They also are physiologically more prone to developing an infection, particularly women. Adolescents and young adults may also lack the resources--money or information--to help them when they develop an STD. Therefore, they may be at high risk for undertreating their STD, putting them at risk to continue its spread to another person.
Remember that for one to become infected with an STD, there has to be sexual contact with an infected partner. When an STD causes uncomfortable symptoms like urethral or vaginal discharge (as in gonorrhea or trichmoniasis), painful urination (also in gonorrhea, or as in non-gonoccocal urethritis) or visible lesions (as in herpes or chancroid), a person is likely to seek medical attention before engaging in further sexual activity. But even STDs that do produce uncomfortable or unsightly symptoms have incubation periods in which they are infectious but asymptomatic (as in herpes or syphilis). They may also be harbored in people who did not complete treatment enough to eradicate the disease, but took enough medicine to alleviate the symptoms. Or, perhaps the infected host did not recognize the illness they had as an STD; this usually happens when the initial symptoms of the disease might be mistaken for the common flu (as in HIV disease or hepatitis). Finally, and most importantly, numerous STDs have the ability to "hide" from the host, like when a person harbors an STD but is completely without symptoms. STDs that remain asymptomatic in a significant proportion of their hosts include chlamydia, genital warts (human papilloma virus), herpes and HIV disease. In these ways, microorganisms survive by developing strategies for easy transmission from host to host.
It is important to remember the basic concepts behind transmission of an STD. To acquire an STD, one has to engage in risky behavior - like unprotected sexual activity. This will put certain populations at higher risk, like adolescents (as stated earlier), persons who abuse drugs and alcohol, and people who exchange sex for money (sex workers). But it is important to avoid stereotyping. Anyone has the ability to acquire and transmit an STD, regardless of his or her gender, race, color, sexual orientation or nationality.
All these concepts of epidemiology of infectious disease make up the basic reproductive number (R ), the expected number of new infectious people that one infectious host will produce during the infectious period of his or her disease in a susceptible population. The infectious host is called a source contact, and the people whom he or she infects are called index cases when they present in a health clinic for help. If an index case has continued to infect others, those new cases are called secondary contacts (or exposed contacts or spread contacts). The formula for the basic reproductive number (R ) helps develop an estimation of new index cases within a population.