A research team at the University of Calgary believes they have discovered why diseases, like arthritis, make people so tired.

Dr. Mark Swain has found that inflammatory diseases like arthritis, inflammatory bowel disease and liver disease, allow so-called "immune" cells to infiltrate the brain and it's those cells that cause fatigue.

Dr. Swain says the fatigue can often be the worst part of a disease and that the next step is to reverse the process which in turn will stop the fatigue.

"I think when people have profound fatigue that's associated with inflammatory disease, they'll often say that it's the most incapacitating thing that they're experiencing from their disease and if they could only get over the fatigue, they could deal with the other aspects of their disease," says Dr. Mark Swain from the University of Calgary.