Fever itself is not life-threatening unless it is extremely and persistently high, such as greater than 107 F (41.6 C) when measured rectally. Risk factors for worrisome fevers include age under 2 years (infants and toddlers) or recurrent fevers lasting more than one week. Fever may indicate the presence of a serious illness, but usually a fever is caused by a common infection, most of which are not serious.