May 11, 2011 | A fever is not always a bad thing. In fact, fever is an important part of the body’s defense against infection. Most bacteria and viruses that cause infections in people thrive best at 98.6 degrees Fahrenheit. Many infants and children develop high fevers with minor viral illnesses. Although a fever signals that a battle might be going on in the body, the fever is fighting for the person, not against them. A fever usually means your body is responding to a viral or bacterial infection. They can also be caused by heat exhaustion or by certain inflammatory conditions such as rheumatoid arthritis. A vaccination or some medications, such as antibiotics and drugs used to treat high blood pressure or seizures, can also raise your body’s temperature. Sometimes it’s not possible to identify the cause of a fever. If you have a temperature of 101 or higher for more than three weeks and your doctor isn’t able to find the cause after an evaluation, the diagnosis may be fever of unknown origin.