...which, of course, is not permission to have risky sex
Syphilis cycles not driven by risky sex
12:11 27 January 2005
NewScientist.com news service
Kelly Young
The rise and fall of syphilis on a decade-long cycle in the US is due to the interaction of infection and the immunity that it induces - not changes in sexual behaviour, a new study shows.
Researchers also found that cycles of syphilis epidemics in US cities have synchronised since the second world war, suggesting an increasingly connected sexual network across the country.
The mechanism underlying the syphilis cycle also occurs in other parasite-host relationships. People infected by the syphilis bacterium often become immune to later infections, meaning infections fall. But when infections reach a low level, less of the sexually active population has immunity and so infections begin to rise again.
In contrast, people do not develop immunity after gonorrhea infections. "Although gonorrhea infects the same people, you really don't see the same cycle at all," says Nicholas Grassly, at Imperial College London, UK, and lead author of the study.
