Immunological theory indicates that infections can prevent allergy by directing the immune reaction against an antigen towards a Th1-type response, thus inhibiting an allergy-associated Th2-type response. The author briefly reviews the evidence indirectly supporting the theory that infections early in life reduce risk of atopy: the protective effect of having older siblings, the increasing risk of atopy with a higher standard of living, and higher prevalence of atopy in Western Europe than in the former communist countries in the East. A new study from Guinea-Bissau is cited, demonstrating an apparently strong protective effect of measles infection in relation to the risk of being skin test positive to common allergens. The medical and ethical questions raised by these findings are pointed out: do we have to choose between measles and an increase in allergic diseases? And if so, who should decide-the individual or society?