Chronic fatigue syndrome is almost as mysterious today as when cases of “yuppie flu” first surfaced in the 1980s. No infectious agent has been found to cause the illness, which is defined as six months of fatigue and at least four flulike symptoms. Some researchers argue that the disease is psychologically induced — perhaps even a form of “mass hysteria.” Others are convinced it has a physical cause. The federal government is funding investigations into possible disturbances in the immune system and the brain. Surprisingly, new studies suggest minorities and blue-collar workers — not yuppies — are most likely to suffer the symptoms. The illness probably will turn out to be a complex interplay of biological, psychological and social forces, many experts agree.