IQ (intelligence quotient) is oft a controversial measure of someone’s smarts — so take this new study with a grain of salt. Researchers from Aarhus University in Denmark have released a new report claiming that infections might be linked to impaired cognitive ability when it’s measured by IQ. In other words, getting seriously ill from an infection might have a negative impact on your brain and ability to think.

The researchers examined 190,000 Danish people born between 1974 and 1994, who had their IQ measured between 2006 and 2012. Among these Danes, 35 percent had been in the hospital for an infection before their IQ test. The researchers found that participants who had been hospitalized with an infection had an IQ score 1.76 lower than average, while people with five or more hospitalizations due to infections had an IQ score that was 9.44 lower than the average.

“Our research shows a correlation between hospitalization due to infection and impaired cognition corresponding to an IQ score of 1.76 lower than the average,” Dr. Michael Erikson Benrós of the National Centre for Register-Based Research at Aarhus, an author of the study, said in the press release. “Infections in the brain affected the cognitive ability the most, but many other types of infections severe enough to require hospitalization can also impair a patient’s cognitive ability. Moreover, it seems that the immune system itself can affect the brain to such an extent that the person’s cognitive ability measured by an IQ test will also be impaired many years after the infection has been cured.”

Immune System And Mental Health

The authors hypothesize that infections can impact the brain through peripheral inflammation — or inflammation of the immune system and central nervous system that has been linked to neurocognitive function.

“Infections have previously been associated with both depression and schizophrenia, and it has also been proven to affect the cognitive ability of patients suffering from dementia,” Benrós said in the press release. “This is the first major study to suggest that infections can also affect the brain and the cognitive ability in healthy individuals.”

Past research also points to a potential link between the immune system and mental health. A study released in 2013 found that inflammation was linked to cognitive aging and increased the risk of dementia. Another study published in JAMA Psychiatry found that children who had high levels of a protein that’s released into the blood in response to infection were at a higher risk of developing depression and psychosis later on.

“Inflammation may be a common mechanism that influences both our physical and mental health,” Professor Peter Jones, an author of that study and the Head of the Department of Psychiatry at the University of Cambridge, said in the press release. “It is possible that early life adversity and stress lead to persistent increase in levels of IL-6 and other inflammatory markers in our body, which, in turn, increase the risk of a number of chronic physical and mental illness.”

Meanwhile, the most recent study is the first to delve into the infection and IQ connection, though past controversial research had identified a link between warmer countries that had increased rates of malaria and other diseases, and lower average IQ. Whether that study is completely flawed or not, in general it’s difficult to measure overall intelligence simply by IQ, especially when in the context of different nationalities, since so many other factors (like poverty and education) must be taken into account. Thus, the researchers will have to do some more work to better understand how infection, inflammation, and the immune system might cause cognitive impairment — and perhaps going farther than simply using IQ as a tool for measure.

Source: Benros M, Sørensen H, Nielsen P, Nordentoft M, Mortensen P, Petersen L. The Association between Infections and General Cognitive Ability in Young Men — A Nationwide Study. PLOS ONE. 2015.