What Are the Causes of an Unexplained Fever?

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  • Written By: Helen Akers
  • Edited By: Jessica Seminara
  • Last Modified Date: 14 June 2019
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The main cause of an unexplained fever is some type of bacterial or viral infection. Other causes include prescription medicines and more serious underlying conditions, such as HIV or cancer. An unexplained fever may be accompanied by other symptoms that resemble a cold or allergic reaction. These types of fevers are usually low-grade and can occur once, intermittently, or chronically.

Individuals can experience fever when they acquire an upper or lower respiratory tract infection. The fever may be slight and raise a person's normal body temperature by only a few degrees. In addition to fever, respiratory tract infections might be recognized by other symptoms, such as excess fluid and mucus in the chest, throat, nose and ear areas.

An unexplained fever is often an immune response designed to help the body maintain proper temperature. Bacterial infections that occur in the body's organs, including diseases such as pelvic inflammatory disease, can result in a fever. While these bacterial infections can be treated with antibiotics, they may result in no obvious symptoms or it might take awhile for noticeable symptoms to emerge.

Serious diseases that compromise the immune system may result in a chronic unexplained fever. One of those diseases is HIV or AIDS. Since the condition slowly attacks the body's immune system and strips it of its capabilities, a fever develops as the body's futile attempt to rid itself of the virus. Any type of cancer can also result in a persistent fever.


Mysterious fevers are somewhat common in children and babies as their systems adjust to surrounding environments. As the immune systems of children develop, they are more likely to catch miscellaneous viruses from other children. Unless the fever is relatively high, it usually does not need separate treatment and will resolve once the infection is treated.

Certain types of prescriptions and drug use may result in fever. Steroids and cancer treatments can cause the body to develop a inflammatory response that results in a fever. Even excessive alcohol and illegal drug use might result in the development of a persistent fever that lacks additional symptoms. Some diseases that develop as a result of alcohol abuse, such as liver damage and scarring, might be the true underlying cause of the fever.

Both children and adults can develop an unexplained fever. The usual cause is an underlying condition that is weakening the immune system in some way. Certain chronic conditions, including diabetes and lupus, can lead to persistent fevers.


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Post 3

@turkay1-- Unexplained fevers in adults can be due to Lyme disease, I know because my cousin had it.

Lyme disease is caused by a tick bite and barely causes any symptoms in the beginning. If someone doesn't notice the bite, there won't be symptoms until weeks or months later and the first symptom is usually an unexplained fever.

After fever, muscle aches and pains, chills, neck and head pain starts.

If it's not treated Lyme disease can also cause meningitis and chronic muscle inflammation and pain. My cousin's was treated very early on so she didn't develop these complications.

Post 2

I've had unexplained fever with no other symptoms several times in the past. I usually don't do anything unless the fever goes above 103 degrees or lasts more than three days.

I think if there are additional symptoms, it's worth seeing a doctor for. But there are so many possible causes of mild fever, that the doctor is not going to do much other than say to take a pain reliever if that's the only symptom.

The only exception is if the temperature goes up too high because high temperature can cause seizures and organ damage. And very high fever is a sign that something more serious is going on anyway.

Post 1

If there is fever, it means that there is a pathogen in the body because this is one of the first mechanisms that our body puts into place to fight diseases. Our body temperature increases to kill as many of the pathogens as possible. So there is always a reason for fever.

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