Science Puzzle
Why Do You Only Catch Chickenpox Once?
Most people who catch chickenpox as a child never catch it again, even when they spend time with someone who has it years later. The virus is just as able to get into their body the second time.
So why don't they fall ill again?
The Answer
The immune system remembers. The second time, it wipes the virus out before it can make you ill.
The first time your body meets a new virus, it has to find the few immune cells, out of millions, whose shape happens to fit that particular virus. Those cells then multiply and start pumping out antibodies, proteins that latch on to the virus and mark it for destruction. All this takes a week or two, and in that time the virus spreads and you feel ill. That's the slow, low curve in Fig. 1.
Once you've recovered, some of those matched cells stay in your body as memory cells, often for the rest of your life. When the same virus turns up again, they recognise it at once and the response is fast and huge: the steep, tall curve. The virus is destroyed before it can multiply enough to cause symptoms. You might never know you'd met it.
Vaccines use exactly this. They show the immune system a harmless version or piece of a germ, so it builds memory cells without you having to suffer the illness first. One twist with chickenpox: the virus doesn't quite leave. It can hide, dormant, in nerve cells for decades and occasionally wake up in later life as shingles, a different and painful illness.
The principle: Immunological memory. After an infection the immune system keeps memory cells that recognise the same germ, so a second encounter brings a faster and much stronger response.