This could explain so very much.  Ben Tufft writes in the U.K. Independent:

A virus that infects human brains and makes us more stupid has been discovered, according to scientists in the US.

The algae virus, never before observed in healthy people, was found to affect cognitive functions including visual processing and spatial awareness. (snip)

Dr Robert Yolken, a virologist who led the original study, said: “This is a striking example showing that the ‘innocuous’ microorganisms we carry can affect behaviour and cognition.

“Many physiological differences between person A and person B are encoded in the set of genes each inherits from parents, yet some of these differences are fuelled by the various microorganisms we harbour and the way they interact with our genes.”

In other words, if you live in a place where these micro-organisms are common, you have a good chance of impaired congnitive function, aka stupidity.  The jokes write themselves.

Actually, the data so far is not conclusive, but certainly worth more investigation:

Of the 90 participants in the study, 40 tested positive for the algae virus. Those who tested positive performed worse on tests designed to measure the speed and accuracy of visual processing. They also achieved lower scores in tasks designed to measure attention.

Hat tip: iOTW Report