Come down with a case of chicken pox and, after you recover, your body seems to wear an invisible suit of armor that protects you from getting the disease again. Catch a cold, on the other hand, and the protective armor seems to fall away quickly.
Common sense indicates that the longer your immune memory lasts, the healthier you will be. Now, a mathematical model indicates that there may be a good reason that you quickly lose your protection against the sniffles. The endless succession of colds that results may protect you from far nastier bugs.
When a person becomes infected by most pathogens, the immune system instantly goes on the attack. After the infection is vanquished, the immune response subsides, but not all the way down to its original level. Long-lived sentries called memory cells remain ready to pounce if the bug reappears.