As many as one in five Londoners had syphilis by their mid-30s during the late 18th century, according to a detailed new study on the sexually transmitted infection (STI) and its spread in the capital of the United Kingdom.

Researchers used data from hospital admissions and workhouse infirmaries to reach their figures, making allowances for duplicate records, private treatments, and the possibility of syphilis numbers getting mixed in with other diseases like gonorrhea or chlamydia.

The findings show a much higher incidence in London than elsewhere in the country at the time – other studies show 'the pox' was half as prevalent in the city of Chester, and up to 25 times less common in rural parts of England and Wales during the late 1700s.

"It isn't very surprising that London's sexual culture differed from that of rural Britain in this period," says historian Simon Szreter, from the University of Cambridge in the UK. "But now it's pretty clear that London was in a completely different league to even sizeable provincial cities like Chester."

"The city had an astonishingly high incidence of STIs at that time. It no longer seems unreasonable to suggest that a majority of those living in London while young adults in this period contracted an STI at some point in their lives."

Together with fellow historian Kevin Siena, from Trent University in Canada, Szreter looked closely at hospital bed occupancy rates and the length of hospital stays at the time, including data from registers dated to the 1760s – and the suggestion is that one-fifth of the population aged 35 and under is actually a minimum estimate.

A total of 2,807 inpatients per year were being treated for syphilis around 1775 in the institutions that were studied, the researchers estimate. Multiplying the findings from their catchment area out to the rest of the city gets to the one in five figure.

This pox prevalence matches up with the contemporary writings of the time, including the diaries of James Boswell – the lawyer recorded 19 incidents of venereal disease in his writings between 1760 and 1786, the result of multiple sexual encounters with prostitutes.

"In an age before prophylaxis or effective treatments, here was a fast-growing city with a continuous influx of young adults, many struggling financially," says Szreter. "Georgian London was extremely vulnerable to epidemic STI infection rates on this scale."

The city was full of young, impoverished women at the time, the researchers point out – pushed to prostitution or other vulnerable positions in order to support themselves.

Syphilis was also rife among many poor, unmarried, immigrant men as well as richer professionals like Boswell (who could pay for more expensive treatments).

Part of the importance of the study comes from the gaps in our current knowledge that it fills – it tells historians much more about how STIs might have played into mortality rates and hospital admissions at the time, as well as the sexual practices of Londoners during the second half of the 18th century.

And anything that experts can learn from the past informs what we do in the present, where sexually transmitted infections like syphilis are still very much with us – and in some parts of the world, on the rise.

"Syphilis and other STIs can have a very significant effect on morbidity and mortality, as well as fertility," says Szreter. "So infection rates represent a serious gap in our historical knowledge, with significant implications for health, for demography and therefore for economic history. We hope that our work will help to change this."

"Understanding infection rates is also a crucial way to access one of the most private, and therefore historically hidden, of human activities, sexual practices and behaviours."

The research has been published in the Economic History Review.