Edward Jenner discovered the cure for smallpox
Edward Jenner noticed the dairy maids do not get small pox. Here he takes a sample of fluid from lesions.
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EGYPT 300 BC
The ancient priests of Egypt, during the rule of the Pharoah kings, knew astronomy and could predict when Nile river would flood and when the farmers can start sowing the seeds. In 300 BC or so, the priests discovered their calculations had a minor error. They could not predict the flooding accurately. Why? Their calendar was based on moon's appearances and movement. Their calendar had 365 days. What was the solution? They added a quarter day to the year and error was corrected.