"A person's immune system is like a library that contains information about every germ ever defeated. If the person encounters the real disease-causing germ in the future, their immune system will know how to defeat it, often before the person experiences any symptoms of illness. This explains why (in most cases) you only catch diseases such as mumps and rubella (German measles) once, even though exposure to the viruses may be repeated throughout life."
Feel free to be the first
Please review the linked page for context.
If you can think of something better than this,
please add it to the database