Sexually transmissible infections are nothing new, but in recent years STI rates in Australia and other developed nations have been rising at an alarming rate.

According to health data from the Kirby Institute, a staggering 18,588 cases of gonorrhoea were detected in 2015, up from 8,388 cases in 2006. In just a decade, the number of gonorrhoea cases annually has more than doubled.

It's a similar story for other infections: in the past decade, syphilis cases have more than tripled, while cases of chlamydia have increased by 43 per cent.

So, what's behind these changes?

More testing means more diagnosis

Experts agree that for chlamydia, most of the increase can be attributed to diagnostic testing.

Advances in technology have created more accurate tests, which have helped to identify more infection.

The number of tests conducted has increased over time as well.

More people testing more often and more accurately means, by default, more infection will be found.

Detecting more infection because of testing is a good thing but it can also make changes over time look more dramatic than they are in reality.

But what might be driving the increases in gonorrhoea and syphilis?

Online dating expands our sexual networks

In the past decade, online dating has had probably the biggest single impact on our sexual lives. Websites and apps designed to facilitate sex and romance are everywhere.

Love them or hate them, online services offer unique features which have interesting implications for the spread of STIs. For example, they can make it easier to have sex with more people.

A recent British study found 35 per cent of sexually active men and 21 per cent of sexually active women reported five or more sexual partners in a year.

Men who found partners online were six times more likely to have five or more sexual partners than those who didn't, and women who dated online were seven times more likely.

This difference is important because the number of sexual partners you have is strongly correlated with your likelihood of getting an STI. More people online means more partners per person, which might mean more STIs.

The question of STIs and finding partners online, however, is a classic "chicken and egg" conundrum: does it actually increase someone's risk, or were they more sexually active even before using the internet?

There is no clear answer yet to this question.

Sex and online dating also make it possible for connections to form between people who might not otherwise have met.

Our social networks tend to form with people who are like us in some way, whether that be via age, class, race, religion or simple geography.

The networks we form sexually — which tend to be extensions of our social groups — are important for understanding how STIs spread.

While people can be highly selective online, the nature of online data can also breakdown traditional social groupings.

If you have sex with someone who is very different from your usual type, it alters the nature of your sexual network. This alteration can impact on the spread of infections, particularly when a network with higher rates of infection overlaps with a network where infection is less common.

Take, for example, gonorrhea in Australia, which is far more common among younger than older gay men.

If someone from a network of mainly older people has sex with someone from a network of mainly younger people, it creates an opportunity for infection to move between two distinct sexual networks.

Over time and with enough points of contact, the chlamydia prevalence in the older, less-infected network will start to look like that of the younger, more-infected network.

This is, of course, only an illustrative example: sexual networks and infection rates are influenced by many factors, of which age is just one.

But in our hyperconnected world, the chance you'll sleep with someone quite different from you — older, younger or something else entirely — is greater than at any point in our history.

What about condoms?

Condoms have always played a key part in preventing the transmission of STIs. Their use (or lack of) is an excellent predictor of infection rates.

The truth, however, is that a great many people do not use them consistently for sex with casual partners.

A recent study of single, heterosexual men in Australia found only 35 per cent used a condom at their last sexual encounter with a casual partner.

There were also notable differences by age: men over 50 years were much less likely to use condoms than younger men. Instead, older men relied on vasectomies as a form of contraception, which, alas, is ineffective for preventing STIs.

Another study found even for younger people who used condoms, nearly half had done so incorrectly, or experienced slippage or breakage.

These issues could be undermining the effectiveness of condoms to protect young people against STIs.

Meanwhile, although gay and bisexual men have historically reported quite high rates of condom use, in recent years this appears to have declined.

A report from the Centre for Social Research in Health found while 68 per cent of gay and bisexual men surveyed in 2013 always used condoms with casual partners, the proportion had fallen to 47 per cent in 2016.

Condom use undoubtedly plays a part in rising rates of STIs — but experts agree it doesn't tell the whole story.

Travel moves infection

Another risk factor for STIs is travel.

In 2016, Australia recorded its highest number of short-term visitors, with more than 640,000 backpackers.

People are also travelling overseas more than ever before.

Travel can move infection from one part of the world to another. But more than that, people caught up in the joys of travel tend to engage in practices that put them at risk for STIs.

This includes the use of drugs and alcohol, having new or multiple sexual partners, and using condoms inconsistently.

Of course, no single explanation perfectly describes what is happening with STIs.

It's probably a combination of these and other factors leading to more infection.

Editor's note (17/08/17): This story was changed to give greater prominence to the paragraph about more accurate testing.

Dr Denton Callander is a social scientist and sexual health researcher at the University of New South Wales. He is also one of RN's Top 5 Under 40 scientists.