IT IS common teaching that an attack of poliomyelitis confers an immunity which is sufficient to protect a person against subsequent infection. The evidence supporting this view is rarely questioned, for second attacks of the disease are infrequent and antibodies against the virus can be demonstrated in the blood of both human and animal subjects. Antibodies can also be detected in the blood in such a large proportion of the population that it is often concluded that most adults have probably been in contact with the virus without development of recognizable symptoms but nevertheless with acquisition of some degree of immunity. The fact that the most susceptible age group consists of young children for whom the chance of previous immunizing exposure has been minimal also suggests the likelihood of some form of acquired immunity in the population at large. In spite of the evidence indicating immunity to be present after