Prevention (CDC) recommends these criteria for diagnosis: Greatly lowered ability to do activities that were usual before the illness. This drop in activity level occurs along with fatigue and must last six months or longer. Worsening of symptoms after physical or mental activity that would not have caused a problem before the illness. The amount of activity that might aggravate the illness is difficult for a person to predict, and the decline often presents 12 to 48 hours after the activity. The 'relapse', or 'crash', may last days, weeks or longer. This is known as post-exertional malaise (PEM). Sleep problems; people may still feel weary after full nights of sleep, or may struggle to stay awake, fall asleep or stay asleep. Additionally, one of the following symptoms must be present: Problems with thinking and memory (cognitive dysfunction, sometimes described as "brain fog") While standing or sitting upright; lightheadedness, dizziness, weakness, fainting or vision changes may occur (orthostatic intolerance) Other common symptoms Many, but not all people with ME/CFS report: Muscle pain, joint pain without swelling or redness, and headache Tender lymph nodes in the neck or armpits Sore throat Irritable bowel syndrome Chills and night sweats Allergies and sensitivities to foods, odors, chemicals, lights, or noise Shortness of breath Irregular heartbeat Increased sensitivity to sensory stimuli and pain have also been observed in CFS. The CDC recommends that people with symptoms of CFS consult a physician to rule out other illnesses, which may be treatable. Onset The onset of CFS may be gradual or sudden. When it begins suddenly, it often follows an episode of infectious-like symptoms or a known infection, and between 20 and 80% of patients report an infectious-like onset. When gradual, the illness may begin over the course of months or years with no apparent trigger. Studies disagree as to which pattern is more common. CFS may also occur after physical trauma such as a car accident or surgery. Physical functioning CFS often causes significant disability, but the degree can vary greatly. Some people with mild CFS may lead relatively normal lives with vigilant energy management, while moderately affected patients may be unable to work or spend much time upright. People with severe CFS are generally housebound or bedbound, and may be unable to care for themselves. The majority of people with CFS have significant difficulty engaging in work, school, and family activities for extended periods of