"It was a bolt from the blue," says Simon. "Alison was my only sexual partner, and I was sure she'd been faithful." Indeed, when they both attended the clinic, Alison's vaginal swab tested negative.
Alison was as shocked as Simon. Although she'd had a number of sexual partners, there was no suspicion that any of them had herpes, and she was appalled to discover that she had probably unwittingly passed it on. "I felt really terrible. Simon was quite ill with it, and although the clinic said I hadn't got herpes, I knew I must have given it to him. They implied he'd picked it up from someone else, but I just knew that wasn't true."
Having lingered in the shadow of Aids for over a decade, genital herpes recently regained the limelight when UK and US studies suggested its prevalence was much higher than previously assumed. As many as one in five people could be infected with herpes simplex virus type two (HSV-II), responsible for the more virulent form of genital herpes, although up to 80 per cent may be unaware they have it. Genital herpes is also caused by the milder HSV-I; while this type usually causes cold sores around the mouth, it can infect the genitals through oral sex.
Official reports put the number of new UK cases at around 15,000 a year. "It's very likely there are many more potentially infectious cases presenting less typically," says Dr Derek Timmins, consultant in genito-urinary medicine at the Royal Liverpool Hospital, and a member of the Herpes Simplex Advisory Panel.
Herpes is a complex and mysterious disease, which transmits in ways doctors are only now beginning to understand. While it was always assumed the virus could only pass on via active sores, recent research uncovered evidence of "silent shedding", where herpes transmits without the presence of blisters; using condoms just during active episodes may still leave partners at risk.
And not everyone has obvious skin lesions; some only experience occasional redness or tingling in the affected area, but can still infect others through oral or penetrative sex. To complicate matters further, the virus can hibernate in the body for years before showing itself, often making it impossible to tell when or from whom you caught it.
The good news is that herpes, while incurable, is generally a mild and benign condition; only an unfortunate few experience recurrent and debilitating episodes, which can be as frequent as every three weeks. No one is sure what causes herpes symptoms to reappear, but triggers include stress, anxiety, fatigue, menstruation and infections like colds and flu. In most cases, however, the first appearance is the worst, and for many it is the last.
However, herpes can occasionally be dangerous for young babies, causing life-threatening encephalitis or brain-swelling. For reasons as yet unclear, neonatal herpes affects more babies in the US than in the UK, where the incidence is just two in 100,000.
"The biggest risk is to young babies in the first 6-12 months of life whose mother is infected with herpes for the first time at or around delivery," says Timmins. "The baby can be infected and its developing immune system is unable to cope. Babies can become ill, even die, if the condition is not recognised and treated promptly."
There are promising signs of a vaccine, and acyclovir and the newer antiviral drugs can shorten the duration and severity of attacks, and even suppress recurrence. But for the majority of people the emotional repercussions of the disease are far worse than the physical. With herpes classed as a sexually transmitted disease, reactions to diagnosis include depression, anxiety, guilt, shame and fear of rejection.
"Unfortunately misunderstandings about the way it's passed on can cause a lot of tension in relationships," says Marian Nicholson, director of support group Herpes Virus Association. "You can get genital herpes from facial sores, but doctors often imply that partners have been unfaithful."
Ten years down the line, Alison and Simon are happily married with children. "Although it was awful at the time, it's not really changed our lives," says Simon, who has not had a recurrence. "We don't think about it now."
The Herpes Virus Association helpline is 0171-609 9061