Unless it's something like Lassa Fever, a fever is the body's natural response to an infection. By raising its temperature, it's attempting to kill off the pathogens. Naturally, conventional medicine views fever as an illness so they can sell you drugs for it. Try to identify the infection, then kill it, and the fever will resolve quickly.
By the way, the fever response is called hyperthermia, and many German clinics deliberately induce it to cure cancers and other intractable diseases.
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