Systemic effects of influenza such as fever, headache and muscle pain result from the response of the immune system to the infection, rather than the virus itself. As Harvard Medical School professor Dr. Raphael Dolin explains in the 2008 edition of "Harrison's Principles of Internal Medicine," the influenza virus has rarely been detected outside of the lungs and airways. Instead, systemic effects are believed to be mediated by chemical messengers such as tumor necrosis factor, interferon, interleukin-6 and interleukin-8 which escape from the respiratory tract and enter the bloodstream. Once in circulation, they issue the physiological equivalent of a distress call, alerting the immune system to the presence of an infection.
Fever
According to a 2009 report in the "New England Journal of Medicine," fever—observed in 94 percent of patients—constitutes the single-most common symptom of influenza. Fever due to influenza usually ranges from 100.4 to 105.8 degrees F and is characterized by a rapid rise during the first 24 hours, followed by gradual abatement during the next two to three days. Rarely does fever last as long as one week. Fever that lasts longer is accompanied by a skin rash or produces febrile seizures should be evaluated by a physician.
Headache
Headache due to influenza may encompass the whole head or be limited to the front of the head, which patients often describe as a constricting band or pressure at the forehead and temples. In some cases, headache may be associated with eye problems such as burning pain, pain with eye movements or pain triggered by light. These effects should not be confused with migraine headaches or eye infections. According to the authors of a 2009 report in the "Journal of Clinical Microbiology and Infections," these headaches usually subside in about three days as other symptoms start to improve.
Muscle Pain
Influenza causes diffuse muscle pain and soreness that may be especially severe in the legs and lower back, says Dolin. Some patients with influenza also experience joint pain or a feeling of weakness, although bedside tests of muscle strength and coordination produce normal findings. For unknown reasons, muscle pain is often the most persistent systemic symptom of influenza. Some individuals, particularly the elderly, may require several weeks to return to previous activity levels.
References
- “Harrison’s Principles of Internal Medicine, 17th edition;” A.S. Fauci, E. Braunwald, D.L. Kasper, S.L. Hauser, D.L. Longer, J.L. Jameson and J. Loscalzo (Eds.); 2008
- “Clinical Microbiology and Infections;” Dynamics of Clinical Symptoms in a Case with Pandemic Influenza A (H1N1); CS Lee and JH Lee; Nov. 13 2009
- “New England Journal of Medicine;” Emergence of a Novel Swine-Origin Influenza A (H1N1) Virus in Humans; Novel Swine-Origin Influenza A (H1N1) Virus Investigation Team; June 18 2009
