| Summary: | Illness is a part of life. For many years, we have known about diseases. But, we were not always so smart. The Black Plague ravaged Europe between 1347 and 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 helped 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.
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