Chronic fever, often also called fever of unknown origin (FUO), is a fever that recurs or does not go away and has no immediate explainable cause, such as an obvious viral or bacterial infection. Such a condition can have numerous causes, and doctors advise people not to ignore this symptom. The degree to which FUO is serious depends on its origin, but since it can suggest serious illness, a visit to the doctor is warranted.
Doctors often look first at one of the obvious causes of chronic fever: infection in the body. This could be relatively hidden and could be due to an abscess somewhere in the body or something like a urinary tract infection. Cat scratch fever, a bacterial infection, sometimes develops symptoms such as chronic fever and swollen lymph nodes, especially in children, and can persist for months. Doctors can often verify the bacterial infection with blood tests or body scans, and through treatment, they can eliminate the fever.
Certain types of viral infections can also cause chronic fever. Patients with undiagnosed chronic fatigue syndrome may have FUO. HIV can also manifest with chronic or recurrent fever.
There are cases where chronic fever is evidence of diseases that suppress the immune system. Conditions like lupus, juvenile and adult rheumatoid arthritis, and HIV/AIDS cause a fairly constant fever. In these cases, the bacterial infection does not need to be actively present, but the body produces a fever in response to what it thinks is a constant assault on its immune system. Alternatively, diseases like sarcoidosis can cause the body to respond with a chronic low-grade fever.
More serious are certain types of cancer that can cause chronic fever. Diseases like lymphoma have an effect on the immune system and FUO could be an early symptom. Other forms of cancer can also cause fever.
Sometimes the body responds to an injury by developing a fever. If a bone is broken or tissue damage occurs, the body may produce a fever as a misdirected healing response and this usually continues until healing is advanced. Alternatively, the source could be another unrelated medical condition, such as heart disease, occasionally blood pressure conditions, or infections in the heart, such as bacterial endocarditis, that compromise its function.
Imbalance in some of the body's hormones can also lead to chronic fever. When people have conditions like hyperthyroidism, where they make too much thyroid hormone, they can get a fever. This condition is easy to verify with blood tests.
A generally more benign cause of chronic fever is a reaction to certain medications. Some people can continue to take medications, but may have a low-grade fever when using certain types. For other patients, drug rush can present serious problems. If the fever is burdensome or troublesome, the medications could be changed or stopped.