If a deadly virus or other foreign invader has invaded your body for the first time, it may take your immune system a few days to destroy it. First, the right lymphocyte (a special type of white blood cell) must be found. The body has millions of lymphocytes to choose from; each one is capable of making a single kind of weapon that will match a particular virus.

Once the right lymphocyte has been found, it reproduces wildly. In a few days the bloodstream is full of these warriors that either latch onto the enemy and destroy it or produce antibodies that inactivate the enemy and mark it for destruction. The antibody attaches itself to the molecules on the virus surface just like a key fitting a lock.

Your immune system has another remarkable ability. Once the right weapon has been found, it remembers it. This means that antibodies can quickly be made in the event of a future invasion by the same type of microbe. That is the reason that a person who has recovered from a childhood disease, such as measles, mumps, or chickenpox, is usually not susceptible to a second attack of this disease. Medical science has worked along with this process to boost the immune system. Vaccinations cause the immune system to produce antibodies against diseases a person has never had before. By this means, children acquire immunity from some diseases. But certain diseases (for example cancer and hay fever) defy man’s efforts to bring them under control.