The sex disease syphilis adapted from a severe, debilitating illness to a milder form in order to survive, research suggests.
Syphilis could produce extreme symptoms
Dr Robert Knell, of Queen Mary's College, London, argues the disease was too virulent for its own good.
Sufferers became so repellent that they were unlikely to have sex. To ensure that they did, and continued to pass on the bacterium, it had to change.
Dr Knell's theory is published in the journal Biology Letters.
Syphilis in its early form caused disfiguring pustules on the face accompanied by a foul smell.
Dr Knell argues this would have been obvious to any potential sexual partners of a sufferer, enabling people to avoid the infected person and thereby reducing transmission.
Other symptoms, such as agonising pains in the joints, would have effectively disabled the sufferer, or at least distracted them from seeking out new sexual partners.
As a result, less virulent strains of the disease were transmitted more often, thus leading to changes in the severity of the disease.
Changes in virulence in diseases introduced to animal populations have been observed before - but this is believed to be the first credible example of such rapid changes occurring in a human disease.
Dr Knell said: "Syphilis changed from a virulent disease to a relatively mild one in a very short period.
"Our use of antibiotics to treat it means that it may still be evolving towards lower virulence.
"Syphilis is rare nowadays but its incidence is rapidly increasing, and in recent outbreaks in the UK some of those infected noticed no symptoms at all.
"This could have serious implications concerning the spread of infectious diseases such as HIV because the chance of contracting HIV through heterosexual sex with someone who is HIV positive is about 30 times greater if you have syphilis."
Syphilis first appeared in Europe in 1496 and was known as the Great Pox or French Disease.
At first it caused terrible sickness, including severe ulceration of the part of the body first infected (often the genitals), pustules, soft tissue being eaten away to the bone, and the rapid onset of "gummy" tumours.
However, within 50 years syphilis changed from an acute, severe and debilitating disease to the milder infection that is modern syphilis.
Dr Knell says the decline in the virulence of syphilis was noted as early as five to seven years from the start of the epidemic - less than a single generation and too short a time for any resistance to the disease to be formed.
This would indicate that, rather than there being any changes in people's immune systems, it was in fact the disease that evolved during this period.