West Nile virus
This mosquito-borne virus is found in many countries throughout the world, including the US, Australia, parts of Canada, parts of Europe, but not the UK, Ireland or New Zealand.
Around 1% of the US population have been infected with West Nile virus. Most people experience no symptoms when catching the virus, but about 20% of people develop a fever, headache, vomiting or a rash, and less than 1% develop encephalitis or meningitis, which presents with neck stiffness, confusion or seizure symptoms.
31% of patients with a history of West Nile virus infection reported fatigue that affected their daily activities. Of those with fatigue, 64% met the CDC case definition for CFS.
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So that equates to around 20% of patients with a history of West Nile virus infection developing CFS.