BBC has an interesting report on the psychological dimensions of flu:
While it seems obvious that we feel under-par when we are ill, Dr Harrison says there is more to it than just physical sickness.
“People have assumed that these are just natural consequences of having an infection,” he explained.
“But it seems that whatever the cause of the infection is, the symptoms are exactly the same.”
In other words, no matter what the type of infection or illness, the psychological feelings we experience are largely the same – meaning it is unlikely to be related to the infection itself.
Tests showed that flu-like infections stimulate a portion of the brain, the subgenual cingulate, that also becomes stimulated in cases of major depression. They say the discovery could, for one thing, lead to treatments for depression, at least when it occurs as the side effect of another medication.