Infections can be passed on even if the penis doesn't fully enter the vagina or the man doesn't ejaculate (come). This is because infections can be present in pre-ejaculate fluid (pre-come).
Even shallow insertion of the penis into the vagina (sometimes called dipping) carries risks for both partners. Using a condom can help protect against infections.
There are many methods of contraception to prevent pregnancy, including the contraceptive injection, contraceptive patch, contraceptive implant and combined pill.
Bear in mind condoms are the only method of contraception that protects against both pregnancy and STIs, so always use a condom as well as your chosen method of contraception.
Find out about the 15 methods of contraception.
Anal penetrative sex
This is when a man's penis enters (penetrates) his partner's anus. Some people choose to do this as part of their sex life, and others don't. Men and women can choose to have anal sex whether they're gay or straight.
According to the third National Survey of Sexual Attitudes and Lifestyles, published in 2013, 17% of men and 15.1% of women had anal sex in the past year.
Anal sex has a higher risk of spreading STIs than many other types of sexual activity. This is because the lining of the anus is thin and can easily be damaged, which makes it more vulnerable to infection.
STIs that can be passed on during anal sex include:
Using condoms helps protect against STIs when you have anal sex.
If you use lubricants, only use water-based ones, which are available from pharmacies. Oil-based lubricants such as lotion and moisturiser can cause condoms to break or fail.
Get tips on using condoms properly.
Oral sex involves sucking or licking the vagina, penis or anus. Some men and women (gay and straight) choose to do this as part of their sex life, and others don't.
According to the third National Survey of Sexual Attitudes and Lifestyles, published in 2013, more than 77% of men and 75% of women had oral sex in the last year.
There's a risk of getting or passing on STIs if you're giving or receiving oral sex. The risk increases if either of you has sores or cuts around the mouth, genitals or anus.
This is because viruses and bacteria, which may be present in semen, vaginal fluid or blood, can travel more easily into a partner's body through breaks in the skin.
Generally, the risk of infection is lower when you receive oral sex than when you give someone oral sex. However, it is still possible for STIs to be passed on.
STIs that can be passed on through oral sex include:
If you have a cold sore and you give your partner oral sex, you can infect them with the herpes virus. Similarly, herpes can pass from the genitals to the mouth.
The risk of passing on or getting HIV during oral sex is lower than anal or vaginal sex without a condom. However, the risk is increased if there are any cuts or sores in or around the mouth, genitals or anus.
You can make oral sex safer by using a condom as it acts as a barrier between the mouth and the penis.
You can use any kind of condom during oral sex. Make sure it has the CE mark or BSI kite mark, which means the condom meets high safety standards.
This is when someone inserts one or more fingers into their partner's vagina or anus. It's not common for fingering to spread STIs, but there are still risks.
If there are any cuts or sores on the fingers, no matter how small, the risk of passing on or getting HIV or other blood-borne infections such as hepatitis B or C increases.
Some people gradually insert the whole hand into a partner's vagina or anus, this is called fisting. Not everyone chooses to do this.
Again, the risk of infection is higher if either person has any cuts or broken skin that come into contact with their partner. You can lower the risk by wearing surgical gloves.
This covers a wide range of items, including vibrators and sex dolls. Any object used in sex can be called a sex toy, whether it's designed for this use or not.
It's important to keep sex toys clean. If you're sharing sex toys, make sure you wash them between each use and always use a new condom each time.
Sharing sex toys has risks, including getting and passing on infections such as chlamydia, syphilis and herpes. If there are any cuts or sores around the vagina, anus or penis and there's blood, there's an increased risk of passing on hepatitis B, hepatitis C and HIV.
Urine and faeces
Some people choose to urinate on a partner as part of their sex life, and others don't. There's a risk of passing on an infection if the person who's being urinated on has broken skin.
Faeces (poo) carries more of a risk. This is because it contains organisms that can cause illness or infection, for example shigella. This is a bacterial infection of the intestine that causes severe diarrhoea and is often mistaken for food poisoning. It can be caught during oral-anal sex and giving oral sex after anal sex when even a tiny amount of infected poo can get into the mouth and cause infection.
Although faeces doesn't usually contain HIV, it can contain the hepatitis A virus. There's a chance of infection when faeces comes into contact with broken skin, the mouth or the eyes.
Cutting the skin – called scarification – as part of sex carries risks. Infections such as HIV, hepatitis B and hepatitis C can pass from person to person through broken skin.
No sexual contact is needed. Simply getting blood on a partner is enough to transmit these infections.
To lower the chances of infection, cutting and piercing equipment should be sterilised and not shared.
Are sex toys safe?
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What should I do if I think I've got an STI?