Sexually transmitted disease

A sexually transmitted disease (STD) is a disease which spreads by having sex. STDs can spread through oral sex or anal sex, or when two people's genitals touch. Today, STDs are usually called sexually transmitted infections (STIs). In the past, they were called venereal diseases (VD).
STDs can be caused by viruses, bacteria, or parasites. Examples of the most common STDs are listed on this page.
Types of STDs
STDs caused by viruses
Human Immunodeficiency Virus (HIV)
HIV causes the immune system - the system in the body that is supposed to fight off infection - to not work as well as it should. This makes people with HIV more likely to get infections and some other diseases, like some cancers. HIV is the virus that causes AIDS.
Human papillomavirus (HPV)
There are about 40 different kinds of HPV that can be spread through sex and affect the anus and genitals. Examples of diseases that HPV can cause include:
Genital warts
Cancers of the anus, vagina, penis, mouth, or throat
Cervical cancer (over 70% of cervical cancers - 7 cases out of every 10 - are caused by HPV)
Hepatitis viruses
Hepatitis is an infection of the liver. There are three common types of hepatitis: hepatitis A, hepatitis B, and hepatitis C.
Hepatitis A causes a short-term liver infection. It can be spread by having sex, though this is not the most common way to get hepatitis A.
Hepatitis B and hepatitis C cause much more serious, long-lasting liver problems. Both can cause very bad liver disease, like cirrhosis, and death. They are more likely than hepatitis A to be spread by having sex.
Herpex simplex virus
The herpes simplex virus (HSV) causes genital herpes. Genital herpes can cause painful blisters on the genitals and anus. Herpes can also cause sores on the mouth.
There is no cure for herpes.
STDs caused by bacteria
Chlamydia
Chlamydia is one of the common STDs in the world. In humans, it is the most common STD caused by bacteria. In the United States alone, about 2.8 million more people get chlamydia infection every year.
If chlamydia is not treated, it can cause serious problems, like diseases of the genitals and eyes. It can even cause blindness. Chlamydia can also cause permanent damage to a woman's reproductive system if it is not treated.
Gonorrhea
Gonorrhea can infect the genitals, rectum, mouth, throat, and eyes. In bad cases, it can spread through the blood to infect the body's joints. In the worst cases, it can spread through the blood and infect the heart (causing endocarditis, an infection of the heart's valves) or the spinal cord (causing meningitis).
Syphilis
Syphilis can cause many serious problems if it is not treated. It makes a person much more likely to get HIV, because it causes sores on the genitals that make it easier for HIV (and other STDs) to get spread. After a few years, if people with syphilis do not get treated, they can have serious problems with the organs in their body. Eventually - without treatment - syphilis infects the brain and causes death.
STDs caused by parasites
Trichomoniasis
Trichomoniasis is the most common STD in the world that is not caused by a virus. It can cause inflammation swelling of the vagina (vaginitis, urethra (urethritis), or penis. It can also cause more serious problems. If it is not treated, it can cause permanent damage to a woman's reproductive system. It can also cause prostate cancer, and makes a woman more likely to give birth to a premature baby.
Prevention
The only way for a person to be sure they will not get an STI is for that person not to have sex. This includes sex of any kind.
There are some things that a person can do to make it less likely to get an STI:
Practice safer sex: Use condoms.