Viruses are sneaky things. When they die, their DNA often sticks around. Some of them transform into freely roaming DNA that evades treatment. Others tunnel into their host’s genome and lie dormant, sometime for decades.

But dormant doesn’t mean gone. When the time is right, they strike. If a patient with hepatitis B virus (HBV) or HIV stops taking antiviral drugs, it can trigger a severe rebound of the disease. Stress, aging, and other factors that dampen the immune system can also reactivate the chickenpox virus, triggering the painful, blistering rash known as shingles.

Now, scientists have found a way to shut down one of these biological sleeper agents. Called epigenetic silencing, the technology hunts down lingering HBV genes in infected mice and adds a chemical tag that switches the genes off, without cutting the viral DNA. Unlike gene editing tools that shred genetic material, potentially causing collateral damage to the host’s genome, the epigenetic approach could be safer but still long-lasting.