In this study of the 50 states, parasite prevalence (measured as the infectious disease rate) is strongly associated with lower mean IQ for all races combined (it explains almost half the variation in mean IQ). The authors interpret this to support the view that a distressed immune system during periods when the brain is developing results in a tradeoff that hurts IQ.
The problem is that, according to their data, the correlation between the infectious disease rate and percent black in a state is .90--the two measures are highly collinear. Childhood infectious disease rates are much higher among blacks, but in the U.S. the number of cases of cholera, measles, meningitis, pertussis, rubella, tetanus, or TB even among blacks is low. If parasite prevalence was a major cause of low IQ, the distribution should not be a bell-shaped curve but a small percentage of low-IQ individuals and a high percentage of people with normal IQs. Parasite prevalence could contribute something to low IQ, but I don't see how it could explain almost half of the U.S. variation in cognitive ability. The researchers should have examined race-specific data.
Parasite prevalence that the authors make such a big deal about looks to be simply a proxy for race.