What caused the spread of polio?
Polio is caused by a virus. There is still some mystery over how the virus is spread. There is a belief that the virus is spread through contract with faeces of an infected person. Before the arrival of modern sewage treatment plants, most people came into contact with the disease and developed immunity to it. Once sewage systems were improved people no longer came into contact with the disease and stopped developing antibodies against it. This theory does not explain why the impact of the disease in America fluctuated and showed no relation to improvements in sewage systems.
