Researchers said they had identified a family of retroviruses in patients with chronic fatigue syndrome, opening up a potentially promising new avenue of treatment for a debilitating disease that afflicts as many as four million Americans and 17 million people world-wide.

The finding will likely spur patients with the condition to seek treatment with drugs used to fight HIV, the virus that causes AIDS. Although HIV and the newly identified virus group are different, they are both retroviruses.

The...