Condoms must be used across the board, regardless of the HIV status, to avoid contracting any sexually transmitted infection (STI), and because people with HIV are not infected with the same strain of the virus (there are types, groups, and subtypes of this virus, check the following link for more information: http://www.avert.org/hiv-types.htm). So if one partner has HIV type 1 and the other has type 2 HIV, they may transmit the viruses to one another. This is known as "superinfection", during which the second infection occurs several months after the first, as the body's immune response to the first virus is sometimes not strong enough to prevent infection with a second strain, especially with a virus belonging to a different subtype.