Cholera
The English engineer Joseph Bazalgette solved the problem for London in the middle of the 19th century. He invented the system for cleaning foul water, a system which is still used world-wide. Cholera has come back to the world because cities have hugely increased populations, and the treatment of sewage has overrun the facilities. It was the British who built most of the world's sewage plants, and until about 1950 they were able to work well unless they were damaged by warfare. Later governments did not invest money in their infrastructure, which meant the increased population eventually caused cholera to return.