Why STIs spread so easily: the surprising maths of sexual partners

Australians report more sexual partners than people in almost any other country. World Population Review puts Australia’s average at 13.3 lifetime partners, the second highest in its rankings. To keep things simple, say everyone has had 13.

Start with a couple: 2 people. Each of them has had 13 partners, so between them they are connected to 26 people. Each of those 26 has had 13 partners too, which makes 338. Keep going back like this, and the numbers grow very quickly:

By 7 steps back, the number is more than four times Australia’s population of 27,921,150 at 31 March 2026. You can try your own numbers in the calculator below.

The real number of different people is much smaller than the table, for a few reasons. People share partners, so the same people turn up in more than one place. Not everyone has had 13 partners: 13.3 is an average, and plenty of people have had fewer. And most people in any network do not have an STI.

But the point of the sum still stands. Sexual networks are big and closely connected, and an infection only needs one connection to pass from one person to the next.

It also helps to know that having sex twice with someone who has an STI roughly doubles your chance of catching it, and a threesome raises your risk too, because you are exposed to two people at once. These are good reasons to think about testing more often.

Put the size of these networks together with how quietly many STIs behave, and it is easy to see why infections spread. Healthdirect points out that you may not be able to tell if your partner has an STI, because some STIs do not cause symptoms. Someone can have an infection, feel fine and pass it on without knowing. Chlamydia is a good example, as our post on why Chlamydia is known as a silent STI explains.

It also helps explain why STIs are so common in Australia. Launching a national sexual health campaign in January 2024, Health Minister Mark Butler said that 1 in 6 Australians will get an STI in their lifetime. The same release says most STIs are treatable, but early detection is essential.

You cannot check every person in the network, but you can check yourself. Healthdirect says a reliable way to know whether you have an STI is to get a test. If anything is found, you can get treated and avoid passing it on. Testing for the most common infections is routine: around three quarters of the bookings we receive in Australia include a Chlamydia test.

Good times to test are after a new partner, after sex without a condom, and as a regular check if you have more than one partner. Our post on how often you should get an STI test goes through this in more detail.

For a broad check, the Full STI Screen tests for Chlamydia, Gonorrhoea, Hepatitis B, HIV and the p24 antigen, Mycoplasma, Syphilis and Ureaplasma. It is done at a clinic with a blood and urine sample. The incubation period is 28 days, so it works from 28 days after any sex you are worried about, and results come within 5 days of your sample reaching the laboratory.

If you are worried about one recent encounter, the Peace of Mind screen can be taken from 14 days after it. It tests for Chlamydia, Gonorrhoea and Syphilis from a blood and urine sample, with results within 3 days of your sample reaching the laboratory. A Syphilis test at 14 days picks up few infections, so a negative result is not definitive until a later test confirms it.

Both tests are confidential. If anything is found, we can help with a consultation and any treatment you need, and our Patient Care Team is available 24/7 on 1-800-750-111 if you would like help choosing.

• World Population Review. Average Number of Sexual Partners by Country 2026. https://worldpopulationreview.com/country-rankings/average-number-of-sexual-partners-by-country
• Australian Bureau of Statistics. National, state and territory population, March 2026. Released 17 September 2026. https://www.abs.gov.au/statistics/people/population/national-state-and-territory-population/latest-release
• healthdirect. Safe sex. Last reviewed December 2025. https://www.healthdirect.gov.au/healthy-living/physical-wellbeing/safe-sex
• Mark Butler, Minister for Health. New sexual health campaign to combat rising rates of STIs (media release). 14 January 2024. https://www.markbutler.net.au/news/media-releases/new-sexual-health-campaign-to-combat-rising-rates-of-stis

Medical review. This article was medically reviewed by Dr Steve Chapman, a doctor with a wealth of clinical experience in the field of STIs, on 9 November 2026. Better2Know content is reviewed for clinical accuracy before publication and updated when guidance or surveillance data changes. It is general information and not a substitute for personal medical advice.