Illness is a part of life. For many years we have known about disease. But we were not always so smart. The Black Plague ravaged Europe between 1347–1352 and killed 30–80 MILLION people. No one knew how people caught the condition and lots of ideas failed–people just died. But not everyone died; possibly their immune systems help them fight off these “invaders”.
Today we know about bacteria, viruses, cancers and other invaders and we usually let them run their course. That is because our bodies detect and fight them. That is immunity.