"In most cases, a serious infection will progress from a low-grade fever to a temperature of 100.5°F or greater within a day," Madsen says. But infections aren't the only cause of low-grade fevers.
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A low-grade fever generally indicates a body temperature that is slightly above normal. There is no formally defined range, but one research article categorized it as between 99.1 F and 100.4 F.