Shingles is due to a reactivation of varicella zoster virus (VZV) in a person's body. Antiviral medications, including acyclovir, are effective at controlling viral reactivation. The reactivation of mitochondria in cancer cells restarts their apoptosis program. Such a regeneration phenomenon, known as multiplicity reactivation, occurs in influenza virus and reovirus. BV may increase the risk of infection with or reactivation of human papillomavirus (HPV). In those with a history of hepatitis B reactivation may occur. Drake demonstrated that poliovirus is able to undergo multiplicity reactivation. Vaccinia virus is able to undergo multiplicity reactivation (MR). Changes in the immune system during menstruation may play a role in HSV-1 reactivation. The periodic fusions permit mutual reactivation of otherwise lethally damaged protocells. Approximately 50% of overt carriers experience acute reactivation. Lonelier people are more likely to show evidence of viral reactivation than less lonely people. That is, multiplicity reactivation can occur. This process, known as multiplicity reactivation, has been studied in several bacteriophages, including phage T4. The specific triggers for reactivation are not well understood.