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The onset of CFS may be associated with preceding stressful events and multiple other precipitants. A study that divided CFS patients into two groups based on whether onset was sudden or gradual found that the rate of concurrent psychiatric disease was significantly greater in the CFS-gradual group relative to the CFS-sudden group. Although both CFS groups showed a significant reduction in information-processing ability relative to controls, impairment in memory was more severe in the CFS-sudden group.
(Source: The Concise Encyclopedia of Fibromyalgia and Myofascial Pain, by Roberto Patarca-Montero, M.D., Ph.D.)