The internet and text messaging are fuelling a practice which involves unprotected sex with strangers in public parks.
Al fresco sex is increasing, say experts
Health chiefs in Kent have taken to posting messages on sites promoting "dogging" - warning of a rise in sexually-transmitted diseases.
They say the craze may be partially responsible for rises in chlamydia, HIV, syphilis and hepatitis.
More than 20,000 people are registered with one UK "dogging" newsgroup.
Dogging is an extension of "swinging" parties - and involves exhibitionist sex in semi-secluded locations such as car parks or country parks.
The location of dogging "events" is now frequently advertised on websites or communicated via mobile phone text message.
Those attracted by these messages may simply act as voyeurs - or be invited to participate.
The NHS Health Promotion team covering the Medway area in Kent noticed that the number of cases of hepatitis rose towards the end of 2002, and some patients said they had probably contracted the illness as a result of dogging.
They decided to post messages on the dogging sites warning of the risks of unprotected sex.
One reads: "Avoid the sex lottery for yourselves and your associates." It recommends NHS-sponsored safe sex websites.
One academic who has carried out a study into dogging suggests that modern technology is leading to a swift rise in the numbers taking part.
Dr Richard Byrne, from the Rural Affairs and Environment Group at Harper Adams University College in Newport, carried out a survey of country park rangers asking them about "anti-social behaviour" in their parks.
He expected their prime concern to be vandalism and fly-tipping, but found that many complained about the rise in the use of their park as a venue for sex.
He said: "One newsgroup about dogging has more than 22,000 registered users - and one we saw which set itself up only a few weeks ago already has almost 700.
"It's a nationwide activity and in the last four or five years it really has grown."
He said that the health threat should not be underestimated: "We've talked to people who do this - they are having unprotected sex with nine or more people in a week.
"The issue of sexual health is a big one."
Legally, the issue of dogging is a grey area - "doggers" are committing no offence unless they are witnessed by a member of the public who can be defined as "outraged" in the eyes of the law.
A new sexual offences Bill currently moving through Parliament may give the police more options to tackle the issue, particularly if it takes place in a location where it is likely that an unwitting member of the public is likely to witness it.
However, ministers say it is not their intention to criminalise outdoor sex in a public place that is sufficiently isolated as to make witnessing unlikely.
And while voyeurism on unknowing subjects is likely to become an offence, if there is consent, there is no offence committed.
Dr Byrne said that because dogging operated at the "fringes of legality and social acceptability", those taking part were at risk.
He said: "There is the potential for younger and more vulnerable people to be drawn in - certainly there is heavy use of alcohol associated with these events, and there is anecdotal evidence of rohypnol (a "date-rape" drug).
"There are concerns that prostitutes will be drawn to areas where dogging is common, and that other types of crime will increase."
He said that, in country parks at least, changes to the design of car parks might help curb their use as venues.
However, he added: "You can't simply increase the amount of lighting - that just makes it easier to make videos or take pictures."