Hepatitis B Virus infections are extremely common around the world. About 257 million people are chronically infected, 80 million of whom live in China alone. As a direct result of their infection with Hepatitis B Virus, quite a sizable number of the individuals will go on to develop hepatocellular carcinoma, one of the deadliest forms of liver cancer. Worldwide, Hepatitis B Virus resulted in 887,000 deaths (2015).
However, research has shown that Hepatitis B Virus has a far higher mutation rate than previously thought. In a surprisingly short period of time, the virus effectively takes over the human cell's replication machinery in order to proliferate billions of viruses per day with every possible mutation represented. With modern sequencing, these mutant viral strains, sometimes called 'quasispecies', can be precisely characterized.
You understand the problem: if a treatment is curative today, it might not be so tomorrow.
 WHO: Factsheet Hepatitis B. See here.
 Lin et al: New insights into the evolutionary rate of hepatitis B virus at different biological scales in Journal of Virology – 2015