Sexually transmitted infections (STIs) can pass from person to person through several types of sexual contact.

Rather than use the older term, sexually transmitted diseases (STDs), doctors now tend to refer to “STIs.”

STIs commonly transmit through penis-and-vagina sex, but they can also pass on via anal sex, oral sex, and, rarely, open-mouthed kissing.

STIs are very common. The Centers for Disease Control and Prevention (CDC) estimate that in 2018, there were more than 2 million cases of STIs.

Some people may not experience more than mild symptoms. A person may not know that they have an STI.

Sexually active adults should undergo regular testing for STIs to keep transmission rates as low as possible. The more a person knows about STIs, the better prepared they are for prevention.

In this article, learn more about some common STIs, including transmission, testing procedures, and treatment options.

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Several types of sexual contact can transmit an STI.

A person’s sex, ethnicity, and age do not make them more or less likely to contract an STI. Anyone who is sexually active has a risk of developing this type of infection.

Some common STIs include:

Symptoms, causes, risk factors, and treatments vary by type of STIs.

Learn more about common STIs with this symptom guide.

STIs can transmit through:

  • direct contact with a lesion or sore
  • close personal contact, in the case of pubic lice
  • exposure to blood that contains the infectious agent
  • contact with vaginal fluid or semen
  • the sharing of needles

There are three categories of STIs:

  • bacterial
  • viral
  • parasitic

Various bodily fluids, such as vaginal secretions, semen, saliva, and blood, contain the bacteria or viruses involved. In some cases, a person can contract an STI by coming into direct contact with fluid that contains the bacteria or virus.

To avoid contact with this fluid during oral, anal, or vaginal sex, use condoms or dental dams.

Other STIs, such as herpes, can pass on via direct skin-to-skin contact, such as through oral, anal, or vaginal sex. It may transmit from the mouth to the genitals, for example, during oral sex.

STIs such as HIV and hepatitis can transmit through contact with infected blood, when sexual partners have open sores, for example, or when people share needles.

Parasitic STIs, such as pubic lice, can transmit through close personal contact, passing from the pubic hair of one person to that of another. They can also transmit via sheets or clothing that has been close to a person’s pubic hair.

Learn more about STIs and oral sex here.

Several factors can increase the risk of contracting an STI, and it is important to avoid them.

To prevent the transmission of STIs, try:

  • Using a condom or dental dam: These can greatly reduce the risk of direct contact with a lesion or fluid that carries an infectious agent.
  • Using water-based lubricants: Oil-based lubricants can cause condoms to break.
  • Undergoing STI tests: Regular testing is important, especially before having sex with a new partner.
  • Getting vaccinations: Having vaccines that protect against conditions such as hepatitis and HPV is a good idea.
  • Avoiding drug and alcohol use: These substances can increase the likelihood of behavior that is risky from a health perspective.
  • Taking extra precautions or avoiding sex with people who use injected drugs: A person who uses injected drugs may be more likely to have STIs such as HIV or hepatitis.

It is also worth noting that STI risk increases with the number of sexual partners. A person who only has sex with a likewise exclusive sexual partner has a lower risk of contracting these infections.

Taking the precautions listed above can reduce the risk of transmitting or contracting STIs, but they are no guarantee.

It is important to speak with a person about their sexual history before having any kind of sex with them. It may be a good idea for new partners to undergo testing before they begin having sex.

Undergoing testing for STIs is a good idea for anyone who has:

  • a new sexual partner
  • had sex with multiple partners
  • any symptoms of an STI

It is worth noting, however, that no STI test is completely accurate. Also, there are no tests available for certain STIs, and some people with these infections do not develop symptoms or do so only after an extended period.

Anyone who thinks that they may have an STI should have a test. If the results are negative, it is a good idea to retest soon.

To check for an STI, a doctor may test the:

  • blood
  • urine
  • other fluids

The best course of treatment depends on the type of STI. Treatment may relieve symptoms, prevent them from flaring up again, or cure the infection.

It is important to remember that an STI can pass on to a partner during treatment.

Some common treatments include:

  • Antibiotics: These treat bacterial STIs, such as chlamydia, gonorrhea, syphilis, and trichomoniasis. People with these infections should complete the full course of antibiotics and avoid having sex until the treatment is complete.
  • Antiviral medications: These treat infections such as herpes and HIV by preventing outbreaks, and some can suppress HIV for a lifetime. However, it may still be possible to transmit these STIs to a partner, so taking precautions is key.
  • Lotions and creams: These can treat pubic lice and help relieve symptoms of sores.

Anyone who may have an STI should contact a doctor. Most STIs require prescription treatment. It is important to follow all guidance regarding treatment and take steps to prevent transmission.

Someone with an STI who does not receive effective treatment may develop:

The best way to prevent complications is to undergo testing and receive any necessary treatment early.

STIs spread through some bodily fluids, skin-to-skin contact, and sometimes, sharing sheets or clothing that have been close to the genitals.

People who are sexually active should undergo regular screening. Early detection and treatment can prevent STIs from transmitting and causing complications, some of which can be dangerous or deadly.

Follow a doctor’s guidance carefully. In many cases, effective treatment can suppress or cure the infection.