Smallpox vaccinations may be a factor here too:
The worldwide eradication of smallpox may, inadvertently, have helped spread HIV infection, scientists believe.
Experts say the vaccine used to wipe out smallpox offered some protection against the Aids virus and, now it is no longer used, HIV has flourished.
The US investigators said trials indicated the smallpox jab interferes with how well HIV multiplies.
But they say in the journal BMC Immunology it is too early to recommend smallpox vaccine for fighting HIV.