Recent studies confirm that this happens easier than we might want to admit.
Viral strains emerge which are resistant to the drugs.
And if you catch their HIV, you won't have many treatment options either. At the Eighth Retroviral Conference in February, one study addressed 14 newly infected people, all infected by the same the source person with MDR-HIV that was resistant against all three classes of HIV meds. Another case described a convict in the Texas prison system. Although HIV-negative when he went in, he became infected with a strain that had two major resistance points. He continued to engage in risky behavior, and within a year he had HIV with nine resistance mutations against all three drug classes.