A team at the University of Birmingham's Centre for Human Brain Health investigated the link between the mental fog and inflammation the body's response to illness.
In a study published in Neuroimage, the team in collaboration with the University of Amsterdam showed that inflammation appears to have a particular negative impact on the brain's readiness to reach and maintain an alert state.
Scientists have long suspected a link between inflammation and cognition, but it is very difficult to be clear about the cause and effect.
"For example, people living with a medical condition or being very overweight might complain of cognitive impairment, but it's hard to tell if that's due to the inflammation associated with these conditions or if there are other reasons," said senior study author Dr Ali Mazaheri from University of Birmingham.
The study focused specifically on an area of the brain that is responsible for visual attention.
A group of 20 young male volunteers took part and received a salmonella typhoid vaccine that causes temporary inflammation but has few other side effects.
Brain activity was measured while they performed the attention tests.
On a different day, either before or after, they received an injection with water (a placebo) and did the same attention tests.
On each test day, they were unaware of which injection they had received.
The results showed that inflammation specifically affected brain activity related to staying alert, while the other attention processes appeared unaffected by inflammation.
"This research finding is major step forward in understanding the links between physical, cognitive, and mental health and tells us that even the mildest of illnesses may reduce alertness," noted Professor Jane Raymond.
The next step for the team will be to test the effects of inflammation on other areas of brain function such as memory.
( With inputs from IANS )