The Health Protection Agency (HPA) is warning of a dramatic increase in the number of cases of syphilis.
If syphilis is untreated it can lead to organ failure and even death
In 1996 there were 137 people diagnosed with the disease in England and Wales but by 2005 that number had risen to almost 3,000.
Similar rises have also been reported in Scotland.
The HPA says the increase is because people are not practising safe sex and that the number of detected cases is likely to be the tip of the iceberg.
In its early stages, the bacterial infection causes ulcers and a rash. But, if left untreated, it can cause organ failure, mental illness and death.
Despite being virtually eradicated in Britain with the discovery of penicillin, increasing numbers of people are visiting their GPs or clinics with the sexually transmitted disease.
In some cases doctors are reported to be seeing people with secondary symptoms of syphilis for the first time in their careers.
The HPA says many people will ignore the symptoms and not seek help - and some may not even know they are infected.