In 2007, April Thompson was running a marathon when exhaustion caught up with her.

From that point on, the 37-year-old Washington resident said she was perpetually tired, plagued by headaches and chronic pain.

When she was still miserable on vacation in North Carolina months later, Thompson visited a doctor and received a diagnosis that put all of her symptoms into perspective: chronic fatigue syndrome.

Today, four years after her initial diagnosis, Thompson said she remains troubled by headaches, extreme fatigue and muscle and joint pain.

"I love to travel but I can't," Thompson said. "I can't walk my own dog; I have to rely on neighbors. I can't get around by myself."

Before falling ill with CFS, Thompson led an active life. She was involved in a number of sports and had just been certified to be a group exercise instructor. CFS has radically changed things for her.

"People have no idea how painful and difficult this condition is," Thompson said. "It's such a kaleidoscope of symptoms. It's unlike anything you've ever experienced."

UNRAVELING A MYSTERY

Thompson is one of the roughly 17 million people worldwide who suffer from CFS, a mysterious disease the validity of which has sometimes been questioned.

But a recent discovery has given sufferers new hope for conquering their disease--and pinpointing its cause.

In a recent study, scientists identified a virus called murine leukemia virus (MLV) in 86 percent of study participants who suffer from CFS. Just 6 percent of non-CFS sufferers carried the virus, according to participants' blood samples.

MLV, like HIV, is a retrovirus, and some researchers hope that the virus can be treated with HIV drugs. Three of these drugs have already been proved to halt replication of the MLV virus in test tubes.

A previous study detected a related virus, XMRV, in blood samples from 67 percent of the CFS patients studied. That study, published last fall in the journal Science, sparked interest in a possible connection between CFS and the family of retroviruses.

However, the new study failed to detect XMRV, as did other previous studies. Lingering questions about the viruses call for further study into which, if any, may be associated with CFS.

MORE TO INVESTIGATE

Elly Brosius, a CFS sufferer and facilitator for the Northern Virginia CFS support group, said she doesn't want to get her hopes up about a breakthrough.