Dave, after turning in my last grant, I was worn out. You know when you are too tired to even sleep?

I know! It's like you feel it in your bones. Imagine feeling that way all the time? It is called Chronic Fatigue Syndrome or CFS, and affects about 17 million people world wide. Generally, it is a prolonged and severe tiredness not relieved by rest.

Early symptoms are similar to common viral infections with muscle aches, headache and fatigue. What makes CFS debilitating is that these symptoms can last six months or more.

Sufferers can have a wide range of problems: Fatigue for more than a day after moderate excercise; forgetfulness; joint pain that can move from joint to joint; and lymph node tenderness.

Chronic Fatigue Syndrome has not been taken seriously because a cause was not known. But recent studies point to a virus.

It's a retrovirus called XMRV. It's the very same virus we told you about in an episode linking the virus to prostate cancer. In a study, researchers looked at the blood of CFS patients and found seventy percent had the virus, compared with just five percent of healthy people.

Now, that doesn't necessarily mean the virus is responsible. CFS patients may just be more susceptible to XMRV because their immune system is suppressed.

Having said that... scientists do believe XMRV may be to blame mostly because it leads to a long-term immune response. Think about how you feel during the 5 days the average flu lasts. Imagine a continuous and vigorous immune response that exhausts the body.

If researchers link XMRV to Chronic Fatigue Syndrome, drugs used to treat HIV may be effective. That is because XMRV is in the same family of viruses.

If the drugs work, they'd bring great relief to patients of a debilitating disease.