John Snow wanted to find the cause of cholera.He believed that people became ill after eating infected food.He used the next outbreak of cholera to test his idea.He studied a small area of houses which got their water from the same pump.Many people in the area died.John Snow showed that this was because the water in the pump was infected.He showed that cholera could be defeated if peple drank clean water.

打的累死了，参考书上的蛤~