No one likes being sick. In addition to the runny nose, sore throat and headache, colds often bring with them an overall feeling of malaise and depression.
But that might soon be a thing of the past. Scientists in Germany think they've worked out what's behind the bad mood.
When a person is infected, part of the body's immune response is the release of a protein called CXCL10. This protein impairs the firing of neurons in the part of the brain called the hippocampus, which is important for learning, memory and, most importantly in this case, mood.
Anti-viral molecules known as interferons used to help fight diseases like multiple sclerosis, hepatitis C and cancer are known to cause similar psychological reactions in patients, including fatigue and insomnia.
Thomas Blank and Marco Prinz of the University of Freiburg gave mice a virus known to cause flu-like symptoms in people. After being infected, the mice were forced to swim, but showed "significantly higher immobility" than those that hadn't been infected.
The scientists interpreted this as "a behavioural sign of despair" and "depressive-like behaviour".
"This depressive-like behaviour required the activation of the CXCL10/CXCR3 signalling pathway," they said in a statement. "CXCL10 released from brains bound to neuronal CXCR3, which, in turn, impaired the activity of neurons in the hippocampus."
The discovery suggests the malaise and fatigue that often accompanies a cold could be minimised, leaving just the sore throat, headache and runny nose to deal with.
"Our findings suggest that preventing the release of CXCL10 or blocking its receptors at an early phase should eliminate at least the initial stages of sickness behaviour seen in response to viral infection or type I interferon therapy," says Prof Blank.
The research is published today in journal Immunity.