Some cancers do cause a fever, although most of the time a high temperature is due to an infection that’s easily managed and runs its course. Fatigue and other symptoms typically arise with infections, too.

Yet fever symptoms are associated with specific types of cancer, leukemia and lymphoma among them. What healthcare providers refer to as a “fever of unknown origin” (FUO) is just one clue toward a diagnosis and treatment.

This article defines what FUO is and some of its possible causes. It details why fever can be an early symptom of leukemia and other cancers.

Fever of Unknown Origin

“Fever of unknown origin” is the term used to describe a fever that meets all of the following criteria:

  • Temperature of 101 degrees F or more
  • Lasts for at least three weeks, either constantly or with multiple episodes
  • Does not occur in an immunocompromised person, such as with HIV infection
  • Has no other identifiable cause, after a physical exam and multiple lab and imaging tests

Keep in mind that an FUO has to be prolonged, so that generally rules out the flu, a routine cold, or many of the other common causes of fever that would be expected to resolve within three weeks.

Possible Causes of FUO

Given that there are hundreds of possible causes of FUO, the list of possible diagnoses—even for fevers lasting longer than three weeks—is quite long. Many of these causes fall into one of three categories: infection, cancer and other diseases, or medication.

FUO doesn’t automatically indicate cancer. There are more than 200 possible diagnoses associated with FUO, some of them quite rare. In up to 50% of FUO cases, the cause of fever is never found. But in 20% to 30% of all FUO cases, leukemia, lymphoma, or colorectal cancer (CRC) are the reason.

Infections That Cause Fever

Many types of infections can lead to fever of unexplained origin. It’s common for nausea, fatigue, and other symptoms to arise with the fever, depending on the cause. Conditions may include:

  • Ear infections
  • Urinary tract infections (UTIs)
  • Gastroenteritis (stomach flu)
  • Pneumonia

Fever associated with an infection remains the leading cause of FUO.

Cancers That Cause Fever

Several different types of cancer may include FUO as a symptom. These cancers include:

  • Lymphomas, both Hodgkin and non-Hodgkin lymphoma types
  • Leukemias, notably acute myeloid leukemia (AML) and chronic lymphocytic leukemia (CLL)
  • Renal cell (kidney) cancers
  • Hepatocellular (liver) carcinoma
  • Castleman’s disease
  • Glioblastoma multiforme, an aggressive brain cancer
  • Ovarian cancer
  • Colorectal cancer

However, a number of other health conditions may begin with infection and/or inflammation. They also may include FUO as a symptom. Among these conditions are:

Early Symptoms of Leukemia

“Leukemia fever” may be one early sign of the disease. Other early symptoms of leukemia include fatigue, enlarged lymph nodes, frequent nosebleeds, unexplained bruising, body pains, night sweats, and weight loss.

  • Appendicitis
  • Atrial myxoma, a benign tumor in the heart
  • Deep vein thrombosis
  • Meningitis
  • Mononucleosis (mono)
  • Autoimmune diseases, like arthritis, lupus, and multiple sclerosis (MS)
  • Tuberculosis

Medications That Cause Fever

Sometimes drugs may be to blame, including certain antibiotics and medications that are taken to prevent seizures. Drugs that can lead to FUO include sulfa-based products, such as stool softeners, and even pain medications like nonsteroidal anti-inflammatory drugs (NSAIDs).

These “drug fevers” are more common if you have taken a drug for years, and less so when it is a new medication. In most cases, the fever goes away within 72 hours if you stop taking the drug.

Why Blood Cancers Cause Fevers

Two kinds of blood cancer, leukemia and lymphoma, are known to produce fevers as an early sign.

Be sure to speak to your healthcare provider about your FUO and any related symptoms before you stop taking any medication you suspect may be the cause.

For example, about 25% of people with Hodgkin’s lymphoma may have what are called “B symptoms.” These include fevers, unintentional weight loss, and drenching night sweats. These symptoms suggest a more aggressive and rapidly growing cancer.

When blood cancers do cause fevers, those fevers can, in some cases, impact the stage and prognosis (or outlook) of the illness.

Summary

There are more than 200 possible causes for why you may experience a fever of unexplained origin, and in most cases, the FUO is not caused by cancer. It’s far more likely that fever is caused by the flu or another uncomplicated infection. Once the infection has resolved, the fever will go away too.

Why Do People Get Fevers With Cancer?

The answer may depend on the type of cancer. For example, lymphoma cancer cells may cause the release of chemicals in your body that cause an intermittent high temperature in the absence of infection. The same principle is believed to be at work with leukemia fevers.

That said, fever is a common symptom in some types of cancer, including leukemia and lymphoma. These cancers of the blood are known to cause a fever in their early stages.

If you have an unexplained fever, be sure to work closely with a healthcare provider to rule out the more obvious causes, such as an adverse drug reaction. It’s important to give a precise medical history and tell them about any other signs or symptoms that arise.

A Word From Verywell

Most FUOs will resolve on their own once the underlying cause is addressed. That said, don’t ignore an unexplained fever, especially one that recurs. It may be an important clue about a serious health issue.

Frequently Asked Questions

  • What are periodic fever syndromes?
  • In rare cases, an unexplained fever is linked to a specific and inherited genetic condition such as familial Mediterranean fever (FMF). Fevers with these conditions usually (but not always) begin in childhood. These disorders are typically not fatal but must be managed across a lifetime.
  • Learn More:
  • What Are Periodic Fever Syndromes?
  • Is a Pel-Ebstein fever also an FUO?
  • Yes. Pel-Ebstein is a fever that cycles every seven to 10 days or so. The fever rises abruptly, stays high for a week, falls, and then stays low for about a week. It’s not common, but a Pel-Ebstein fever pattern may be a symptom of Hodgkin lymphoma.
  • Learn More:
  • Pel-Ebstein Fever Diagnosis and Patterns
  • What therapies are there for cancer fever treatment?
  • Fever caused by cancer often will stop when the cancer is treated. However, treatment itself (especially chemotherapy) may cause fever too. This is likely related to infection and considered a medical emergency your healthcare provider needs to address.
  • Learn More:
  • Side Effects of Chemotherapy

In rare cases, an unexplained fever is linked to a specific and inherited genetic condition such as familial Mediterranean fever (FMF). Fevers with these conditions usually (but not always) begin in childhood. These disorders are typically not fatal but must be managed across a lifetime.

Yes. Pel-Ebstein is a fever that cycles every seven to 10 days or so. The fever rises abruptly, stays high for a week, falls, and then stays low for about a week. It’s not common, but a Pel-Ebstein fever pattern may be a symptom of Hodgkin lymphoma. 

Fever caused by cancer often will stop when the cancer is treated. However, treatment itself (especially chemotherapy) may cause fever too. This is likely related to infection and considered a medical emergency your healthcare provider needs to address.