Germs travel between people. Sex is one of the roads.

Start here: these are just tiny living things

• Your body is home to trillions of bacteria, viruses and fungi. Most are good neighbours. You need many of them.
• Germs move between people in different ways. Cold viruses use the air. Stomach bugs use food. Some germs only survive in warm, wet body tissue, so they need close contact to move. Sex is very close contact.
• That is all an STI is. It is biology. It says nothing about whether a person is clean, careful or good.
• It is also common. More than a million curable STIs are picked up around the world every day, and most cause no symptoms.

Two kinds of trouble

Natural does not always mean harmless. It helps to split the germs into two groups.

Herpes and HPV sit in between. They are travellers, but so common that they are close to a normal part of adult life: about 64 in 100 people under 50 carry herpes type 1, and nearly everyone who has sex meets HPV unless vaccinated.

How to read the table

• The five routes. Kiss is mouth to mouth. Oral is mouth on genitals or anus. Vaginal and anal mean penetration. Skin is touching without penetration, including fingers and shared toys.
• Chance per act means one act of vaginal sex, without a condom, with a partner who has the infection and is not being treated. "Her" is the partner with a vagina. "Him" is the partner with a penis.
• There is no single number for kissing, oral and penetration combined. Nobody has measured that. The figures are for penetration, where the data are. Oral sex is usually lower. Kissing is lower again, except for herpes, glandular fever, CMV and mpox.
• Each number carries a tag. "Measured in couples" was counted in real people. "Model estimate" was calculated from population data. "Per partner" is the share of partners infected over a relationship. Where the table says there is no good number, there is none.
• Small chances add up. A 1-in-5 chance per act becomes about 3-in-5 after four acts.
• Your real risk is usually lower. These figures assume the partner has the infection. If you do not know, multiply by the chance that they do.
• ☠️ Can be fatal. This mark sits on infections that can kill an otherwise healthy adult if nothing is done: through organ failure, cancer or a collapsed immune system. All of them can be prevented, and most can be cured or kept under control when caught early. Infections without the mark can still be dangerous for a newborn or for someone with a weak immune system. The last column says when.
• Treatments follow current US CDC guidance. Other countries differ. The right drug and dose change with pregnancy, allergies, the part of the body infected and local drug resistance. A clinician decides; this page explains.

The table: 28 entries, from very common to very rare

Scientists know of more than 30 germs that can pass during sex. These are the ones that matter in practice, plus three residents that are often mistaken for STIs.

STI or STD?

• Infection means the germ is in your body. Disease means it is causing damage or symptoms.
• Most STIs never become a disease. They stay silent, and they are cleared or cured first. That is why health agencies now say STI.
• "Disease" is not wrong. It is the later chapter. Pelvic inflammatory disease, AIDS and cervical cancer are diseases caused by infections left untreated.
• One line to remember: every STD started as an STI, but most STIs never become an STD.
• The practical point: do not wait for symptoms. The silent infection stage is the stage to catch.

How to protect yourself

What does not protect you

• Washing or douching afterwards. Douching raises the risk of BV.
• Peeing after sex. Good for bladder infections, no proven effect on STIs.
• The contraceptive pill, an IUD, or pulling out.
• A partner who looks healthy or says they feel fine.