What causes a sexually transmitted infection (STI)?

You can get an STI by having intimate sexual contact with someone who already has the infection. You can't tell if a person is infected because many STIs have no symptoms. But STIs can still be passed from person to person even if there are no symptoms. STIs are spread during vaginal, anal, or oral sex or during genital touching. So it's possible to get some STIs without having intercourse. Not all STIs are spread the same way.

This answer is based on source information from National Women's Health Information Center.
Sexually transmitted diseases are caused by a variety of organisms. Human Papilloma Virus (HPV) causes venereal warts and cervical dysplasia, a precancerous condition of the cervix. Herpes Simplex Virus (HSV) causes genital herpes. Gonorrhea (a bacterium) and Chlamydia Trachomatous may cause cervicitis or even pelvic inflammatory disease (PID), a severe intra-abdominal infection that can cause pain and infertility. There are other causes as well and other types of STDs. It is important to avoid STDs by being careful to either avoid having intimate contact with someone who harbors an active STD and use condoms to decrease your chances of becoming infected.
Terrie Watkins
Midwifery Nursing
Sexual activity with an infected partner causes sexually transmitted diseases. STD's are viruses, bacteria or parasites that may be transmitted by oral, anal, vaginal or physical contact. STD's may be life threatening diseases that are incurable or infections that are treatable and curable. It is impossible to know if a potential partner is infected or not by appearances- many diseases do not have obvious signs and the person may not even know they are infected. STD's do not respect gender, age or status-kings, the elderly and children may become victims. Safe sex practices include using a condom at every sexual encounter which may decrease disease transmission.
Angela Lowery
Family Medicine
You can get sexually transmitted infections by having unprotected sexual intercourse with someone who is infected. Not all STIs have symptoms. Some STIs are transmitted even when there are no symptoms present. You can get STIs through vaginal, anal or oral sex.
Paula Greer
Midwifery Nursing

This is a simple answer, short an sweet. STD's come from exposure to an infected partner. Practice safe sex at all times.

You can get sexually transmitted infection by following factors:

Biological factors
Women can catch an STI from infected men more easily than men can from women, because germs can get inside females’ bodies more easily. Women may not even realize they have been exposed. Once germs are inside, the warm, moist body provides a good place for them to grow.

A young woman's cervix is not fully developed until the late teens, and it may be more vulnerable to infection, yet many young women engage in sexual activity earlier than their late teens.

After menopause, the vaginal lining gets thinner and possibly drier, so older women can get small breaks in the skin during sex that leave us open to infection. Health care providers are also less likely to give older women prevention messages and screening tests. Menopausal women may even think their risk is lower at their age.

If women have sex exclusively with other women, the chances of getting an STI are significantly lower, but women can transmit STIs to other women. Many who consider themselves lesbian may also have had sex with men, recently or in the past. During those contacts, the risk for infection is the same as for women who usually have sex with men.

Social and cultural factors
Women, who are fifteen to twenty-four years old, sexually active with more than one partner, or living in an urban setting where the number of people with STIs is greater are at the highest risk.

For many of women, poverty may contribute to a higher risk of getting an STI. Not having enough money can mean lack of access to prevention and treatment, economic dependence on a partner who may be exposing them to infection, or just being preoccupied with day-to-day survival more than anything else.

Some experiences in childhood, including emotional and sexual violence, increase the likelihood of exposure to an STI then or later in life. If the cultural tradition values passivity and submissiveness in women, it may be difficult to refuse unwanted sex and to negotiate safer sex protection. If the culture discourages women from touching or looking at their genitals, they may not notice early signs of an STI.
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.