A virus that changes (mutates) over time may become resistant to certain types of medicine. This resistance may affect future treatment options. This study will compare the amount of virus in the blood of HIV-infected patients who have been monitored for viral load with the amount of virus in the blood of patients who have not been monitored for viral load. For patients who have detectable virus, the type of resistance (mutations) of the virus will be determined by comparing the components of the virus with that of a virus that is known not to be resistant.