The combination of fever and back pain can be worrisome for a patient, since it is one of the patterns doctors look for when ruling out dangerous disorders such as cancer or infection. Fortunately, the most common cause of fever and back pain together is simply a coincidental occurrence of their most common individual causes, neither of which is a serious medical problem.
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The most common cause of back pain is musculoskeletal, that is, a strain or sprain of the muscles, tendons and ligaments brought on by excessive or unusual exertion. The most common cause of a fever is a viral upper respiratory infection such as a cold or flu. It's not surprising, then, that the most common cause of fever and back pain is simply when both of these things happen to occur at the same time. In addition, one of the symptoms of the flu is heightened perception of muscle and joint pain, so it can cause greater awareness of back pain that was previously ignored. Musculoskeletal back pain rarely leads to long-term problems, and is best treated by rest, hot compresses and aspirin or ibuprofen. Colds and flu are equally routine, and are best treated by rest and increased fluid intake.
While cancer that originates in the spine itself is extremely rare, it is common for cancers that begin in other organs to spread, or metastasize, to the spine. Cancers that frequently lead to spinal metastasis include breast, lung, prostate, thyroid, kidney and lymphomas. While these cancers are usually detected before they metastasize to the spine, occasionally back pain from metastasis is the first symptom that prompts a patient to seek medical attention. In a paper published in the May 1988 "Journal of General Internal Medicine," 0.66 percent of 1,975 patients seeing a doctor primarily for back pain were found to have underlying cancer. Cancer of any sort is usually accompanied by constitutional symptoms such as fever, unintentional weight loss and fatigue. Doctors screen for these symptoms when evaluating a patient with back pain, just to be sure that a previously undetected cancer is not the cause. They also look for signs of damage to spinal nerves, such as numbness in the inner thighs and genital region, since such damage is more likely to result from a tumor metastasis then from other causes of back pain.
Bacterial infections are another more dangerous, although less common, cause of back pain. Again, infections that arise in the spine itself are uncommon, but in some cases infections starting at other sites can spread there. In addition, infections in other abdominal and pelvic organs, such as the kidney or bladder, can cause pain that radiates to the lower back. Since bacterial infections are often accompanied by fever, doctors will look for potential sources of infection when evaluating a patient with fever and back pain. They will look for other symptoms, such as painful urination or blood in the urine, and may order a white blood cell count if such symptoms are present.