Fever is very common with cancer, but it more often happens after cancer has spread from where it started. Almost all people with cancer will have fever at some time, especially if the cancer or its treatment affects the immune system. (This can make it harder for the body to fight infection.) Less often, fever may be an early sign of cancer, such as blood cancers like leukemia or lymphoma.

There's a reason that your doctor doesn't jump to the possibility of cancer right away. For example, if you’re otherwise healthy, it’s in the middle of flu season, and you develop a fever along with a sore throat, nasal congestion, cough, headache, and fatigue, it's overwhelming more likely that you have influenza.

But there are times when fever, whether alone or in combination with other symptoms, is a part of something else, and your doctor is trained to pick up on clues that indicate the need for a closer look. This ties into an exploration of what's known as a "fever of unknown origin." Remember, though, that the vast majority of fevers seen by doctors can be traced to non-malignant causes.