Christopher Wanjek is the author of the health books "Bad Medicine" and "Food At Work" and the novel "Hey, Einstein!" a comical nature-versus-nurture tale about raising clones of Albert Einstein in less-than-ideal settings. His column, Bad Medicine, appears regularly on LiveScience.
Viruses are tricky beasts. Some of these "submicroscopic" pathogens can "go to sleep" inside a person's body, essentially hiding from the immune system indefinitely, only to reactivate and cause illness later.