A new paper in Science , by Michael Mina et al, strongly suggests that measles messes up your immunological defenses for two or three years. This is the likely explanation for the fact that measles inoculation causes much greater decreases in child morbidity and mortality than you’d expect from preventing the deaths directly due to measles infection. The thought is that measles whacks the cells that carry immunological memory, leaving the kid ripe for reinfections. I think there can be a similar effect with anti-cancer chemotherapy.
If correct, this means that measles is much nastier than previously thought. It must have played a significant role in the demographic collapse of long-isolated peoples (such as the Amerindians). Its advent may have played a role in the population decrease associated with the decline of the Classical world. Even though it is relatively new (having split off from rinderpest a couple of thousand years ago) strong selection for resistance may have favored some fairly expensive genetic defenses (something like sickle-cell) in Eurasian populations.
We already know of quite a few complex side effects of infectious disease, such the different kind of immunosuppression we see with AIDs, Burkitt’s lymphoma hitting kids with severe Epstein-Barr infections followed by malaria, acute dengue fever that requires a previous infection by a different strain of dengue, etc: there may well be other important interactions and side effects, news of which has not yet come to Harvard.