One of the most important and difficult jobs in science is to ask the right question of nature. The right question leads to the design of those experiments or tests which yield the pieces that are required to make a logical pattern out of an incomprehensible jigsaw puzzle. My paper will present more questions than answers, and I hope that among the many that have been asked those questions may be selected that will guide physicians to what must be learned in the future if the origin and evolution of epidemics of poliomyelitis is to be understood and if physicians are ultimately to know what, if anything, can be done to prevent them. From the point of view of public health the chief importance of poliomyelitis as a disease lies in its paralytic manifestations, and the main problem in the epidemiology of poliomyelitis concerns the cause or causes of the