Susceptibility to viruses

Increased susceptibility to viral infections with prolonged recovery periods is a diagnostic criteria for myalgic encephalomyelitis in the International Consensus Criteria. and recurrent flu-like symptoms to a diagnostic criteria in the Canadian Consensus Criteria for ME/CFS (although this doesn't restrict the cause to viruses).

Prevalence
De Becker et al. (2001) found that 69% of people meeting the Fukuda criteria and 80% meeting the older Holmes criteria for chronic fatigue syndrome had recurrent flu-like symptoms, although this may not have been due to frequently catching viruses.

Symptom recognition
Increased susceptibility to viral infections with prolonged recovery periods is a diagnostic criteria for myalgic encephalomyelitis in the International Consensus Criteria (ICC) in the immune, gastro-intestinal & genitourinary impairment symptoms section. A separate ICC diagnostic criteria is flu-like symptoms may be recurrent or chronic and typically activate or worsen with exertion which may result from causes other than new viruses, for example persistent enterovirus infection or immune system abnormalities.

The Canadian Consensus Criteria include the diagnostic criterion of recurrent flu-like symptoms as one of the possible immune manifestations criteria.

Notable studies

 * 2001, A definition-based analysis of symptoms in a large cohort of patients with chronic fatigue syndrome - (Full text)

Possible causes
Increased susceptibility to viruses or frequent infections (including bacterial infections) which take a long time to recover from may also be a result of malnutrition, which may be caused by a poor diet, conditions causing maladsorption of nutrients, or altered nutritional needs caused by certain illnesses (e.g., cancer, or HIV).