Viruses are iconically challenging to define, but they have DNA so they evolve. In the 1980s, the Human Immunodeficiency Virus (HIV) shook the world and propelled a massive undertaking to study the evolution of drug resistance in viruses. On world aids day, December 1st, 2017, we spoke with Dr. Pleuni Pennings from San Francisco State University about how viruses adapt to treatments through time.