Read the text and fill in the gaps with the correct option.

Cholera is a disease spread by dirty water, either when people drink the water or eat food which has been washed in the infected water ... . Unfortunately, until the mid-19th century, people believed that the disease travelled through the air. In the crowded cities of the 19th century in Great Britain, ... . Most doctors ...
that fresh air and a better diet would prevent the disease. Huge fires were ... to drive away the infection, and the clothes of victims were also burnt. The streets were ... . However, people continued to ... . In 1854, a doctor in London, John Snow, ... that cholera spread through infected drinking water. He did this ... about the infection in one part of London. He managed to prove that in the area where deaths from cholera were highest, ... for cholera infection. Improvements made to sewers and supplies of drinking water later led to a decrease in deaths from cholera.