It is probably even harder to learn from experience about immunization, because it does not fix an existing problem, but rather protects against potential future problems. When a child is immunized against measles, that child does not get measles. But not all children who are not immunized actually contract measles (especially if others around them who are the potential source of infection are immunized), so it is very difficult to draw a clear link between immunization and the lack of disease. Moreover, immunization just prevents some diseases—there are many others—and uneducated parents do not necessarily understand what their child is supposed to be protected against. So when the child gets sick despite being immunized, the parents feel cheated and probably resolve not to go through with it again. They may also not understand why all the different shots in the basic immunization regime are needed—after two or three shots, parents might feel that they have done what they should. It is all too easy to get misleading beliefs about what might work in health.

Abhijit Banerjee and Esther Duflo, Poor economics: A radical rethinking of the way to fight global poverty, New York, 2011, p. 73