Persistent low grade fever is also very common. Muscle soreness or weakness, sleep disorders, either over sleeping or insomnia, swollen, tender lymph nodes are all common symptoms. A reoccurring sore throat and headaches also occur. Long-lasting malaise following physical exertion is another common side effect of chronic fatigue syndrome. The cause of chronic fatigue syndrome is unknown.
Researcher have threes and are investigating a variety of possibilities.
Some believe that CFS is due to a combination of viruses, allergies, and hormonal imbalances. Studies also suggest that it could be due to a dysfunction of the immune system. However current theory holds that CFS may develop when an opportunistic virus or other agent invades the body at a time when the immune system is already suppressed. Factors that could contribute to this would be physical, emotional, or environmental stress, or a combination of the three. Some people are skeptical about chronic fatigue syndrome however.