Can Severe Childhood Infections Affect IQ? What Later Research Found

Editor's note: this post is dated September 2011, but the research summarized below was published later — the Danish study in 2015 and the Swedish study in 2018. This article was rebuilt from the later findings during our editorial review.

Could severe childhood infections be linked to cognitive ability years later? A large Danish study suggests an association — with important caveats about what association does and doesn't mean.

The short answer: Researchers tracking 161,696 young Danish men found that those with hospital contacts for infections before conscription testing scored on average 1.76 IQ points lower at age 19 than those who didn't — 1.13 points lower after full adjustment for confounders. The more infection types, the bigger the gap — and brain infections showed the largest effect. (PLOS ONE)

What did the study actually find?

The study, led by Michael Eriksen Benros and colleagues at the University of Copenhagen and Aarhus University, used Denmark's national registers to follow 161,696 male conscripts born between 1974 and 1994, tested for cognitive ability at an average age of 19.4. About 35 percent had at least one hospital contact — inpatient or outpatient — for infection before testing.

The headline numbers:

• Any hospital-treated infection: 1.76 IQ points lower on average (1.13 after adjusting for parental education, birth weight, psychiatric history, and other confounders)
• Five or more infection types: 9.44 IQ points lower on average
• Brain (CNS) infections: the largest effect, 2.38 points lower

The pattern was dose-dependent: more infection types, more recent infections, and more severe infections each meant lower scores.

Does infection cause lower IQ?

The study shows an association, not proof of causation. The researchers proposed that severe infections may trigger an intense immune response — a "cytokine storm" — that can affect the developing brain. Notably, the association appeared to fade with time: the longer ago the last infection, the smaller the gap, suggesting the brain partially recovers.

A later Swedish study of 647,515 men, published in 2018, reported a similar link between infections in childhood and cognitive ability at conscription — and found that the first year of life appeared to be a particularly sensitive period.

What about everyday childhood illnesses?

This is the reassuring part for parents: the study only covered infections serious enough to require hospital treatment. A routine cold, a mild stomach bug, or a standard childhood fever is not what this research is about. The takeaway is about severe infections — and, indirectly, about the value of preventing them.

What should parents take from this?

Nothing to panic about — and one thing to act on. Keep up with childhood vaccinations, treat serious infections promptly, and don't skip follow-up care after a hospitalization. Protecting a child's body, it turns out, also protects the developing brain.

This article is general information, not medical advice. Talk to your pediatrician about any concerns regarding your child's health.

Frequently asked questions

How big is a 1.76-point IQ difference? Small for any individual — about a tenth of a standard deviation. It matters at the population level, not as a prediction about any particular child.

Do children recover? The study suggests partial recovery over time: more recent infections were linked to larger gaps, implying the brain heals, at least in part.

Does this apply to girls too? The Danish conscript study covered only young men, so the findings can't be directly generalized. The later Swedish study likewise examined conscripts, so the question for broader populations remains open.