There is no clean-cut answer, but here are some things to consider. First, there are different types or "strains" of the HIV virus. A person who's been infected with one strain can get infected with a second strain when exposed to it (like through unprotected sex). When this happens, it is referred to as a "superinfection." It is not yet clear how often superinfection occurs. Someone whose first HIV infection is being treated may become sicker once the second strain of the virus enters the system. This happens because the new strain may be more difficult for the immune system to control, or because the new strain is resistant to the HIV medications the person is taking. (See resistance testing for an explanation of what it means for the virus to be resistant.) As mentioned above, little is known about superinfection--how often it occurs, what problems can result. More studies are currently underway to sort this out. In the meantime, HIV-infected individuals will have to decide whether they are willing to take the risk not only of becoming superinfected, but possibly superinfecting someone else. There are more reasons for two HIV-positive people to avoid unprotected sex. Unprotected sex allows the transmission of other sexually transmitted diseases (STDs), such as gonorrhea, chlamydia, and syphilis. Nearly all STDs are curable, but syphilis can be more complicated to treat if you have HIV. Also, there may be a small risk of acquiring hepatitis C through unprotected sex, but researchers are only now beginning to study this. Even if a person is taking antiretroviral medicine and has an undetectable viral load, it's unclear whether this lowers the risk of infecting someone else through sex. A very large international study is going on right now to find some answers to this question, but we won't know the results for several years. | 
