You do get stupid(er) when you are sick

When you are sick, your mind feels a bit slow. But you ignore it since you know the cold virus infects the lining of the airways and not the brain. 

Except that appears to be false. You actually do get dumber when you are sick:

This theory could explain why we lose our mental edge when we get sick, Kipnis says. When we’re healthy, T cells keep the immune cells in the meninges from inflaming the brain. But when we get sick, the T cells loosen their hold to let the immune system attack invading pathogens. The resulting inflammation helps clear out the invaders, but it also blunts learning. When we’re sick, Kipnis proposes, it’s more important to launch a powerful immune attack than to have a sharp mind.

More here.