The virus that causes chicken pox can cause the human body to produce memory cells: After having the chicken pox once, a re-exposure to the virus is fought off by the memory cells, preventing a person from contracting the disease more than once. But in this case, you may not get the “chicken pox,” but another problem: After a chicken pox infection, the virus remains dormant in the body’s nerve tissues. While the immune system keeps the virus at bay, later in life—usually as an adult—the virus (it is called herpes zoster, from the varicella zoster virus) can reactivate (although the reason is unknown, it is often traced to stress or a suppressed autoimmune system). This causes a different form of the viral infection called shingles—which leaves a person with an often painful skin rash.