Reactivate means to make active again.
One example of reactivation is herpes. After the herpes simplex virus infects the skin, it becomes dormant. Sometimes an exposure or condition causes the virus to become active again and cause another rash.
Another example of reactivation is shingles. After someone has chicken pox, the varicella virus also becomes dormant. Something can trigger the virus to become active again, and it can cause the shingles rash.
Sources:
Finch, Roger, Dennis Maki, Allan Ronald. "Varicella-Zoster Virus Infections." Infectious Diseases, 2nd edition. Ed. Jonathan Cohen, et al. New York: Mosby, 2004. 125-9.
"Reactivation." Mosby's Medical, Nursing, and Allied Health Dictionary. 6th ed. 2002: 1469.
Yeung-Yue, Kimberly. "Herpes simplex viruses 1 and 2." Dermatologic Clinics 20(2002): 1-21.
