A fever of unknown origin is a fever of at least 101 degrees that lasts for three or more weeks, with no clear cause.
Though the cause is sometimes found eventually, many fevers get better without doctors ever knowing what caused them. 
Possible causes of fever of unknown origin include: 
<ul>
 <li>Infections.&nbsp;</li>
 <li>Connective tissue disorders, such as rheumatoid arthritis. </li>
 <li>Cancer.</li>
 <li>Drug reactions.</li>
 <li>Inflammatory bowel disease.</li>
</ul>
In some cases, the cause of the fever is unknown because it's fake. Factitious fever occurs when a person artificially heats their thermometer or takes in substances that raise their body temperature. 
According to the <a target="_blank" href="http://www.netreturns.biz/healthinfo/BounceLink.aspx?Shortcut=nih">National Institutes of Health</a>&nbsp;and other experts, you should call a doctor if an adult or child has a fever of 105 degrees or higher, unless it comes down easily with treatment and the person is comfortable. For infants younger than 12 weeks, call a doctor for any rectal temperature of 100.4 degree or higher. A doctor should also be called if a fever persists for more than 24 hours in a child under 2.
Reviewed 9/7/2023

What is a fever of unknown origin?

Most fevers come with other symptoms, together signaling illness. But sometimes fevers appear on their own and the cause can't be found.