Treatment of some diseases can suppress the immune system. This can cause other conditions to reactivate. Recent cases have shown that hepatitis B virus (HBV) reactivates in people who had already recovered from it during treatment for chronic hepatitis C (CHC). Their treatment was direct-acting antiviral (DAA) agents. Researchers want to see how common this reactivation is. They want to learn what the effects are. They will study data that have already been collected.