There were a lot of discussions about this when I developed a chicken pox like rash some months back. As I'm on medication which suppresses my immune system I rang my GP, was called straight in, was there for an hour, sent to the hospital and had the quickest admission ever to A&E and was in hospital for 3 days on anti-virals while they tested my blood. Turned out I'm immune to chickenpox, I had it badly as a child, but they still took the action above to check, just in case.