What causes unexplained fever?

Fever is often the body's response to infection, but it can also accompany inflammatory conditions, a reaction to a medicine, or some blood cancers. In people whose white blood cells are low, an infection may worsen quickly and may not cause obvious local symptoms. Acute leukaemias, lymphoma, and childhood cancer can sometimes present with fever, but fever alone does not identify a cancer. The cause is assessed using the timing, other symptoms, medicines, examination, and blood tests.

Record when the fever began, how it was measured, associated symptoms, recent medicines, travel, sick contacts, and any cancer treatment. Follow your oncology service's fever instructions and check before taking fever-reducing medicine, because it may hide a worsening infection. Assessment may include blood counts, cultures, urine testing, imaging, or other tests chosen from your symptoms. Growth factor support may be discussed when low white-cell counts are part of the problem.

If you are receiving cancer treatment or have been told your white-cell count is low, contact your oncology team urgently for a measured fever or chills, even if you otherwise feel well. Seek emergency help for confusion, severe breathlessness, fainting, blue or grey lips, severe weakness, or rapidly worsening illness. People not in treatment should arrange prompt assessment for persistent or recurring fever, unexplained weight loss, drenching sweats, swollen lymph nodes, or unusual bruising.

What temperature or symptoms should trigger an urgent call? Could my medicines or treatment lower my infection defences? Which tests are needed? Should I keep a thermometer and symptom record? When should I use the emergency department?