Do Infections Make You Dumber? - Pacific Standard

Do Infections Make You Dumber?

A Danish study suggests a link between severe infection and lower IQ scores.
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"Darn the cucumbers! Never mind the doctor, send for the minister." (Photo: Boston Public Library/Flickr)

"Darn the cucumbers! Never mind the doctor, send for the minister." (Photo: Boston Public Library/Flickr)

Getting sick sucks. There's the sore throats, the fever, brain fog, sometimes the gastrointestinal distress. But that's not all: Severe infections might slightly injure your intelligence as well, according to a recent study of Danish military conscripts.

The conventional—or at least popular—view of illness is that you get sick, stay in bed for a while, and then get better. Unfortunately, it's often not so straightforward. Particularly with severe infections, there's a risk of sepsis, a sort of immune-system freakout that leads to inflammation throughout the body, which could affect the brain. According to a 2010 study, elderly patients who survive sepsis are 10 percent more likely to suffer lasting, severe cognitive impairments. Even without sepsis, a number of recent studies suggest that a person's cognitive abilities can suffer as a consequence of HIVCMV, and other major infections.

Those records revealed a 1.13-point drop in IQ associated with having a childhood infection severe enough to land someone in the hospital.

Yet most of the studies on infections' long-term effects on the brain focus on special populations, such as the elderly or people with compromised immune systems. (Transplant patients, for example, are particularly at risk of CMV infection.) But what about everyone else? What about, say, strapping young Danish soldiers?

It's not that Michael Benros and his colleagues at the University of Copenhagen and Aarhus University were particularly interested in young men's intelligence and medical history. It's just that the Danish draft board maintains records on 161,696 of those young men born between 1974 and 1996. They also happened to give all of them an IQ test in 2005 or 2006. And since the Danes use the same ID numbers for conscription and hospital admissions, the researchers could link the results of the IQ test to a soldier's medical history—in particular, whether they'd been hospitalized for severe infections as children.

Those records revealed a 1.13-point drop in IQ associated with having a childhood infection severe enough to land someone in the hospital. For the average person, that corresponds to moving from the 50th percentile in IQ to about the 47th; not a huge change, but nothing to sniff at either. Though the effects declined with time since hospitalization, there was a lasting impact—even 15 years later, soldiers who'd been hospitalized for an infection still had a 0.9 point IQ deficit. The effect got stronger the longer the hospitalization, too: A stay of longer than a month corresponded to a drop of 3.45 IQ points.

Of course, the results are limited to otherwise healthy young men, and they don't imply that infection causes the IQ drop. Still, Benros and his colleagues write that that is the likeliest explanation. For example, the biggest drops in IQ score were associated with central nervous system infections, which by definition are the most likely to cause inflammation in the brain. Something to think about next time you're home sick.

Quick Studies is an award-winning series that sheds light on new research and discoveries that change the way we look at the world.

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