(a) A field investigation into an outbreak of poliomyelitis in Auckland in the summer of 1947–8 is described.

(b) The object of the investigation was to study the background of the epidemic in the general population, to discover how it had commenced, and how much minor illness related to poliomyelitis had occurred (i) amongst household contacts, (ii) in other persons residing in the same neighbourhood, and (iii) in those parts of the urban area least affected by the epidemic.

(c) Conclusions reached have been detailed in the preceding section. A relationship was established between minor (‘suspect’) illnesses and positive cases; the ratio of ‘suspect’ to positive cases at different age levels has been calculated, and an estimate is given of the extent to which the population as a whole was affected. Reasons are adduced for believing that, during an epidemic, mode of life and personal habits may be of more importance in determining the fate of the individual than previously acquired immunity.

(d) It is suggested that indoor dust-borne infection, at school and in the home, may be important, and that the results of recent research into threadworm infestation may throw light on this factor.