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What causes sexually transmitted infections (STIs)

An STI (sexually transmitted infection) is everyone's worst nightmare. Unfortunately, there are many wrong ideas about how you can get an STI.

We’re discussing what causes STIs, and how you catch them. We will also debunk some infamous myths about them. Keep reading to learn more to have fun and stay safe.

Medically reviewed by Dr. Anand Abbot MRCGP Written by our editorial team Last reviewed 06-10-2023

What causes an STI?

Bacteria, parasites or viruses generally cause STIs. The infection is then spread from person to person via bodily fluids (e.g. semen or blood) or physical contact.

How an STI is spread is based on the type.

Bacterial STIs

Bacterial STIs include:

The bacteria lives in the mucous membranes of the vagina, penis, rectum or mouth. This means they are most commonly spread through bodily fluids.

Viral STIs

Viral STIs include:

Unlike bacterial infections, a virus can exist outside the body which means it can spread in numerous ways.

Parasitic STIs

Parasitic STIs include:

  • pubic lice (crabs)
  • trichomoniasis

Like viruses, the parasites can exist outside the body and therefore can spread in different ways.

Can I get an STI without having sex?

While sex is the main way to catch an STI, there are other ways of catching them that don’t involve penetrative or oral sex. This is because STIs can be spread through any kind of bodily fluid or physical contact, which isn’t always through sex.

We will discuss many other ways to catch an STI that doesn’t involve sex.

How do I catch an STI?

There are 4 main ways to catch an STI.

Graphic showing 4 principal ways to catch an STI

Unprotected sex

The first is unprotected sexual intercourse. This can be intravaginal sex, anal sex or oral sex. The STI can be passed on through bodily fluids (e.g. semen or vaginal fluids) or from skin-to-skin contact.

Sexual contact and kissing

An STI can also be spread through sexual contact, where your hands or genitals touch the genitals of someone with an STI. Certain STIs, such as oral herpes (cold sores), can also be transmitted through kissing.

Pregnancy and childbirth

You can also pass an infection onto your baby through pregnancy and childbirth. This happens in 3 main ways:

  • through the placenta to an unborn baby (e.g. syphilis and HIV)
  • during childbirth through the birth canal (e.g. gonorrhoea, chlamydia, hepatitis B and genital herpes)
  • through breast milk during breastfeeding (e.g. chlamydia, gonorrhoea, trichomoniasis, genital herpes, genital warts, HIV, and syphilis)

These are certainly the most common ways to catch an STI. However, some STIs can be spread in specific ways.

What are some other ways to catch an STI?

There are some more uncommon ways to catch an STI. Depending on the type, several factors can increase the risk of transmission.

needles-icon Sharing needles or razors - Both instruments can cut or pierce the skin, which means there is a chance for people to mix blood. This puts you at a higher risk of HIV and Hepatitis A, B or C.
unwashed-sex-toys Sharing unwashed sex toys - Any blood, pus from sores, parasites or fluid on an unwashed sex toy can transmit HIV, Hepatitis and Syphilis.
contaminated-surfaces Contaminated surfaces - Dirty sheets, towels, surfaces and clothes can give you a sexually transmitted infection (STI). Pubic lice and trichomoniasis are two kinds of STIs that are spread this way.
blood-transfusion Blood transfusion - It is rare but possible to get HIV from a blood transfusion. Hospitals and blood donation centres have ways to make sure this does not happen.
contaminated-food Contaminated food - Hepatitis can be spread through food that has been touched by an infected person.

These causes are much less common. However, you should still be careful to prevent the risk of any infection.

Can I catch an STI from sharing cutlery?

Generally, you cannot catch an STI from sharing cutlery or drinkware. However, there is an extremely low risk of catching oral herpes (cold sores) through saliva on these items. That being said, you are more likely to catch herpes from kissing or sex than from cutlery or drinkware.

Can I catch an STI from a toilet seat?

It is unlikely you will catch an STI from a toilet seat. It is impossible for bacterial STIs as the bacteria cannot live outside the body.

While some viruses, like human papillomavirus (HPV), can live for some time outside the body, contracting them from a toilet seat is not possible.

You can get parasitic STIs from a toilet seat but it is very unlikely. It is much more likely to catch them from having unprotected sex.

Do condoms prevent all STIs?

Condoms prevent infection 95-98% of the time. However, some STIs cause symptoms that cannot be covered by a condom.

Man reaching for condom in back pocket

Genital warts, genital herpes, syphilis, and parasitic STIs cause symptoms all over the genitalia. Men can get symptoms in the groin, testicles and perineum. Women can get symptoms in and around the labia.

This means that physical contact with the genital area can spread the infection.

How do I prevent STIs?

The best way to prevent an infection is to wear a condom correctly.

If you’re using a male condom, make sure that:

  • the condom fits correctly
  • you take it off whilst the penis is still hard - taking it off while it’s soft can cause semen to leak out
  • you put it on early enough to prevent contact with precum
  • it has been stored correctly
  • there are no tears or damage before using
  • use the correct lubricant to not damage it
  • it has not expired

If you’re using a female condom, make sure that:

  • it is inserted correctly
  • you insert it before any contact with a penis
  • there are no tears or damage before using
  • use the correct lubricant to not damage it
  • it has not expired

Do not use male and female condoms together as this can cause tearing.

Blood test sample for STIs

You should also prevent transmission through regular testing. If you are sexually active and have multiple sexual partners, you should get routinely checked at a sexual health clinic. Many have remote testing options if you don’t want to go in person.

You should also be open with your partner about your sexual history.

What happens if I get a positive test result?

The clinic will advise you on your next best steps. Usually, you will need to get treatment.

Need STI treatment?

Find out more

You can order your treatment online at HealthExpress. You don’t need to wait for a doctor’s appointment or in pharmacy queues.

Simply choose the prescription treatment you need, complete a consultation, and we send you the treatment once approved by a doctor.

Main takeaways

The bottom line is that you can catch STIs in several ways. Anywhere between sexual intercourse and physical touch can spread infections.

The best way to prevent infection is to use a condom and get regularly screened, even if you have no worrying symptoms.

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