Smallpox and AIDS appear to infect cells by the same pathway. Study Suggests Smallpox Vaccine May Fight Aids:
A team at Virginia’s George Mason University said they had shown, in lab dishes, that blood cells from people vaccinated against smallpox were four times less likely to become infected by the AIDS virus.
[...]
A study published in 1999 showed that a relative of smallpox, called the myxoma poxvirus, uses the same cellular doorway — the CCR5 receptor — to infect a cell as AIDS does.