One day when he was five years old Peter developed red, itchy spots on his skin. The following day the spots looked like blisters. Peter had caught chickenpox. Many of the other children in his kindergarden got chickenpox too, but why didn’t any of the adults catch it?
When adults don’t catch chickenpox it is because they have already had the disease as children. The immune system can remember diseases it has encountered before, so new attacks by the same virus are turned back so quickly that we don’t even have time to notice that we have been infected. The first time a virus attacks it takes some time for the immune system to mobilize the troops. In the case of chickenpox this leaves time for the patient to develop the rash and blisters that are typical for the virus.
Chickenpox is caused by varicella zoster, a virus in the same family as the herpes virus. Once you have been infected with chickenpox the immune system doesn’t clear the infection entirely, and the virus remains in the body, hidden in sensory nerves in the brain and spinal cord. This is part of the reason that adults don’t get sick from catching chickenpox again: their immune system’s memory cells are constantly being reminded that the virus is still present in the body.
When the immune system is weakened, for example in connection with leukemia, the memory cells become less efficient and the chickenpox virus may reemerge from the nerve cells where it has been hiding and become active again. Instead of developing blisters all over their body the patients will only get blisters in the area corresponding to the nerves where the virus remained hidden for years. Because sensory nerves are affected by the reemergence of the virus the patient suffers almost unbearable pain. For that reason this disease is called “Hell’s fire” in Norwegian. The English name for it is shingles, for the girdle-like pattern the rash makes on the body.
Memory is one of the immune system’s most important features. Immunological memory makes it possible to protect ourselves against diseases that we have not had yet by vaccinating ourselves against them.
Blogpost by Anne Spurkland 25th November 2014
Translation edited by Alisa Dewan, 26th February, 2015
Originally published in Norwegian 07/09/12