Live Clean -> Die Young

This study is rather interesting in that it parallels some fo the assumptions that polio developed in the mid-50s in the U.S. because of the access of the general population to sanitation:

" The belief that the polio virus is spread by contact with the feces of an already infected person has been offered as an explanation for the increased incidence of polio in developed countries such as the United States during the 20th Century. According to this theory, before the advent of modern sewage treatment plants and other improvements in public sanitation, virtually all individuals were exposed to the polio virus early in their lives when they were at least partially protected by maternal antibodies. Thus, they developed mild, non-paralytic infections, probably during infancy, which provided them with lifelong immunity. However, with better sanitation, both these early infections as well as the likelihood of receiving antibody protection decreased, resulting in greater susceptibility to paralytic polio. Thus, in the words of Smith:

Put simply, paralytic polio was an inadvertent by-product of modern sanitary conditions. When people were no longer in contact with the open sewers and privies that had once exposed them to the polio virus in very early infancy when paralysis rarely occurs, the disease changed from an endemic condition so mild that no one knew of its existence to a seemingly new epidemic threat of mysterious origins and terrifyingly unknown scope (p. 23).
This central theory regarding the spread of polio is supported, at least to some extent, by experiences in third world countries. During World War II, for instance, U.S. and British troops stationed in undeveloped countries were much more likely to contract polio than native peoples, who apparently had already developed immunity (Paul, 1971). Even in the 1970's, when individuals from developed countries came into contact with those from a country without a modern sanitation system, the incidence of paralytic polio was about twenty times greater for those from the developed country (Nathanson and Martin, 1979).
The above explanation for the transmission of polio is generally accepted and seems quite logical. However, the incidence of the disease in the United States during the epidemic years was very irregular, not only from year to year, but from area to area, apparently showing no relation to improvements in sewage treatment. The actual reason for this variation remains another of the polio mysteries. However, it has been suggested that this variability was possibly due to increased virulence of certain virus strains or the presence of environmental conditions that enhanced the disease's transmission (Nathanson and Martin, 1979). "