To the Editor: In Natural history of infectious disease,1 Macfarlane Burnet emphasised that poliomyelitis was the one infective disease of advanced countries that had failed to respond to improvements in the standard of living, leading to epidemics in the first half of the 20th century that increasingly involved young adults, who developed severe paralyses. There was also an abnormally high death rate among young adults. However, in tropical countries where the poliovirus was endemic, the virus spread among infants “with a minimal number of paralytic cases”. Burnet therefore defined a “safe” period in the first year of life, when infection was not likely to lead to paralysis.
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