People have a kind of memory called immune memory  There are very few self-sacrificing cells in human blood, which live for a very short time, and their main function is to destroy as many enemies of the human body as possible  Lymphocytes react quickly to bacteria or simple toxic substances that invade the bloodstream to produce antibodies that stick to harmful substances and prevent them from entering other organs  The task of destroying the enemy falls on the phagocytes in the blood  These cells can easily distinguish foreign cells, remember them and retain memories
Once a person has measles or scarlet fever, he will gain immunity for life
People have immune memory, as long as they have measles and scarlet fever, they will be immune for life