Considerable data are available which indicate that poliovirus infections are readily communicable under natural conditions in the general population. The age distribution of clinical cases of the disease in previous years has resembled that of measles1 closely. This suggests that inapparent infections are widely disseminated, thus producing similar patterns of increasing resistance to both of these infections among older persons. This similarity, together with more direct methods of measurement,2,3 may indicate that from 60 to 1000 silent infections occur in proportion to each recognized clinical case and that the frequency of clinical expression may vary with the age of the persons infected.
In addition to the evidence for widespread infection with the three recognized immunologic types of poliovirus, the socioeconomic level4-8 appears to influence the dissemination of these viruses. Evidence of previous infection with poliovirus has been found to be more prevalent under conditions of poor hygiene