Key Points:

Gangster Al Capone died of it. So did 19th century German philosopher Friedrich Nietzsche. Some say King Henry VIII was afflicted by it. As perhaps was reclusive billionaire Howard Hughes and Ugandan dictator Idi Amin.

It was called the "French disease" in Italy and Germany, and the "Italian disease" in France. The Dutch called it the "Spanish disease", the Russians saw it as Polish, the Turks said the cause was Christian and the Tahitians blamed the British. But what's most troubling about syphilis, the sexually transmitted disease that traces its origins to mediaeval times, is that it's making a comeback - in the same developed nations which, thanks to penicillin, all but wiped it out after World War II.

The "great imitator" - beginning with a shortlived painless sore (chancre), next presenting deceptively as a rash, fever, or sore throat and later progressing to damage the heart or brain and sometimes causing death - is back. What comes with the resurgence is a two-sided puzzle. Why are a significant proportion of syphilis cases showing up among men who have sex with men? And how is syphilis finding its way into the heterosexual population?

The worldwide trend is playing out in New Zealand. Cases of syphilis had remained low for the past 25 years with the national rate at 3 per 100,000 in 1977. But around 2002 sexual health doctors in Auckland became concerned about the number of new cases presenting at their clinics. The numbers were still small - but 40 cases of infectious syphilis in Auckland alone between January 2002 and September 2004 was twice as many as the preceding four years and enough to ring alarm bells.

Voluntary reporting of infections from some clinics and laboratories to Environmental Science and Research added to the concerns. Between 2002 and 2006, syphilis cases increased from 48 to 71, and there were 22 more cases in 2006 compared with 2005.

What frustrated people like Dr Sunita Azariah and Dr Nicky Perkins at Auckland Sexual Health was that they knew this wasn't the full story. Syphilis is not yet a notifiable disease in New Zealand and laboratories processing tests are not yet required to report the disease's incidence.

The pair, who are about publish their first findings in the New Zealand Medical Journal, analysed laboratory data for Auckland from July 2006 to July this year.

"What we found was a large increase in numbers, particularly last year and this year," says Perkins. "And quite a number of those were never getting to the sexual health clinics and therefore wouldn't get reported."

Their research, which for the first time looked at information from GPs who had been treating syphilis, showed an under-reporting of the disease of about 30 per cent. It also showed two distinct groups - 40 per cent of infections were among men who have sex with men and 50 per cent among heterosexuals. For the last 10 per cent there was no information.

But it was how the infection was being acquired that was surprising. The majority of men who have sex with men got their infection in New Zealand, while just over half the heterosexual group got theirs overseas. What's also clear is the disease is now established.

"In both the men who have sex with men and the heterosexual populations, it has gained a foothold - so we're getting local transmission which is a big concern," says Perkins.

The increase in heterosexual cases is more easily explained by international travel and migration, with Fiji, which has a high prevalence of syphilis, as New Zealand's greatest source of offshore infection.

But the increases among men who have sex with men is more problematic - the only obvious explanation being that the safe sex message, long established in the gay community, is frequently being ignored. The question is why - a question that's being asked in a number of circles because with syphilis, HIV is growing, as are gonorrhoea (1047 cases in 2006) and chlamydia (8083 case in 2006).

New Zealand's HIV data, which is collected by the Aids Epidemiology Group, shows parallels with the syphilis trends uncovered by Azariah and Perkins. Of the 70 men who have sex with men diagnosed with HIV in 2006, 74 per cent were infected in New Zealand. That's in marked contrast to the 85 heterosexuals (40 men and 45 women) diagnosed with HIV in 2006, 82 per cent of whom were infected overseas.

Once again the heterosexual increase can be explained by travel and migration - in particular because 2006 was the first year those applying to live in New Zealand were tested for HIV. Of the 49 people diagnosed with HIV through immigration medicals, 71 per cent were by a heterosexual infection and 12 per cent by men who have sex with men. The main ethnic group was African (61 per cent, followed by Asian (22 per cent).

But as with syphilis the increase in HIV infections among men who have sex with men is mostly happening here.

"For about 20 years, with consistent condom use for anal sex, a lot of the traditional sexually transmitted infections that gay and bisexual men suffered from, such as gonorrhoea and syphilis, literally disappeared," says Douglas Jenkin, New Zealand Aids Foundation national campaigns co-ordinator. "That suggests in various ways the traditional safe sex culture is breaking down."

Jenkin says everyone is scratching their heads trying to work what is going on. Some say today's treatments (highly active antiretroviral therapy) for HIV are so much better that men no longer see HIV as such a big threat. Others point to the internet as expanding people's social networks - providing, as Jenkin puts it, "a whole new playground of sexual opportunity and sexual risk". Then there's the view that when safe sex was introduced over 20 years ago men thought it would be a temporary measure when in fact nothing has come along to replace it. Jenkin notes that older men from 35-45 age group are over represented in both HIV and sexually transmitted infection statistics.

"You would think that group of men would know better, having been around when men died quickly they would have first-hand experience of the epidemic." The problem, he says, is that men are experimenting with ways to deal with the threat that aren't foolproof and involve room for error.

The main strategy is "serosorting" or "serodating" which mainly involves HIV-positive men connecting with other HIV-positive men - "poz meets and has sex with poz". From a strictly epidemiological point of view, the strategy makes sense because it keeps the virus within the HIV-positive population. Socially it makes sense too: because everyone is positive, no one has to fear rejection. The problem occurs when seropositives decide there is no need to restrict themselves to safe sex practices. Not surprisingly, anecdotal evidence indicates that where serosorting occurs, the incidences of sexually transmitted diseases, especially syphilis, rises dramatically.

Jenkin says serosorting is fraught with danger, not only because of the difficulty of knowing one's serostatus, but also because it exposes participants to a greater number of the thousands of strains of HIV and compromises the effectiveness of antiretroviral drugs. "If you're infected by someone taking those drugs, they may not work for you - you can become resistant to them."

The combination of HIV and ulcerative diseases like syphilis also affect immunity - making HIV about four times easier to transmit and receive and making syphilis and other sexually transmitted infections progress faster.

But while serosorting is mostly a consensual high risk negotiation with the reality of always having to use a condom, its most extreme form involves wilful recklessness and deliberate harm.

In the Melbourne Magistrates' Court Michael John Neal, 48, is to stand trial for trying to infect 16 men with HIV between October 2000 and March 2005. Five of the alleged victims have since tested HIV-positive. In committal hearings in March the court heard testimony that Neal had unprotected sex with hundreds of men in a single year and was intent on "breeding" the deadly disease. It was also told of "conversion parties", where HIV-positive men would supposedly have unprotected sex with HIV-negative men trying to contract the virus, a phenomenon known as "bug chasing".

Such cases can also result in bizarre defences. In Adelaide an HIV-positive man appealed his conviction for having unprotected sex with three women by insisting there was no clear evidence the virus caused Aids. The defence team wanted to use testimony from "Aids dissidents" known as the "Perth Group" who argued HIV was not a retrovirus and could not be transmitted by sexual intercourse. A supreme court judge threw out the claims as "implausible" and in September sentenced the man, Andre Chad Parenzee, to nine years jail for the reckless transmission of HIV.

Dr Nigel Dickson who heads the Aids Epidemiology Group at the University of Otago Medical School would like to see more links between the prevention of HIV and other sexually transmitted infections. "When you're talking about trying to stop heterosexual men and women getting sexually transmitted infections you should maybe talk about HIV at the same time," says Dickson. "And when you're talking about HIV to gay men you should also be thinking about syphilis and other infections."

He says what's missing in the data on infections like syphilis is whether they are occurring among people with same sex, opposite sex partners or both. Vital to controlling such infections is reducing the prevalence of the disease in New Zealand. But to do that requires the number of infections being treated to exceed the number of new cases occurring. The problem, as Dickson points out, is that we simply don't know what the prevalence of untreated sexual transmitted infections is. And as many sexually transmitted infections exist without symptoms, only testing and treating those seeking care is not enough. He advocates wider monitoring and surveys to properly establish levels.

By June next year the Ministry of Health intends to address some of the problems with data collection by making syphilis, gonorrhoea and chlamydia notifiable diseases and making it compulsory for all laboratories to anonymously report incidences of the infections. But while the lab form information will include basic age and ethnicity demographics it would not include whether the infection was via same sex or opposite sex partners. Public health medicine adviser for the Ministry, Alison Roberts, says gathering such information was costly and that asking questions about sexual behaviour might deter some from coming forward for treatment.

It's slow progress too on universal HIV screening tests for all pregnant women, which was announced in 2005, but so far is only available in the Waikato, where women are tested for rhesus factor and antibodies, syphilis, hepatitis B and rubella, and HIV.

The programme enables the use of modern medicines to reduce the risk - from 30 per cent to 3 per cent - of a HIV-positive mother transmitting the virus to her baby. Sexual health doctors lament the slow rollout and wonder why there is not a more co-ordinated approach - such as also testing all pregnant women for chlamydia and gonorrhoea which can also be passed on to the baby.

Jenkin says the challenge for the Aids Foundation is to reach out to the younger gay and bisexual men who haven't had 20 years of safe sex messages. "What we're missing is groups of men who don't test and sometimes they're the men taking the most risk."