Viral and bacterial infections are the source of most acute (short-duration) fevers. Examples of viral infections include colds and influenza or chickenpox. Bacterial infections include strep throat, meningitis, pneumonia, Rocky Mountain spotted fever (spread by wood tick bite), sepsis (infection of the blood) and urinary tract infections. Other sources of acute fever include:

• Heat stroke. An abnormal rise in body temperature that can cause a high and potentially fatal fever. Heat stroke is one form of fever that qualifies as an illness in and of itself. It occurs in situations where the body becomes overheated. Examples include a child who stays outside too long on a hot day or an infant left in a hot car.
  
  
• Ear infections (otitis media). Common in children, ear infections may cause a fever along with pain, difficulty hearing and sometimes diarrhea.
  
  
• Immunizations. Some children may develop a low-grade fever after they have a vaccination shot, such as diphtheria, tetanus and pertussis or pneumococcal vaccines.
  
  
• Medication use. The use of medications such as antibiotics and antihistamines sometimes can cause fever as a side effect.
  
  
• Overdressing an infant. Very young children may get fevers if they are dressed too warmly in a hot environment because they cannot properly adjust body temperature.
  
  
• Teething. A low-grade fever sometimes occurs when an infant is teething.
  
  
• Poisoning. Chemical pneumonia accompanied by fever can result from inhaling hydrocarbon chemicals found in substances such as gasoline or furniture oil.
  
  
• Temporal arteritis. This inflammation of an artery in the head can cause a fever.

Chronic fevers also are caused by a number of different conditions. For example, viral infections that occur in sequence (such as a series of colds) can cause recurrent fever. Chronic or relapsing infections (such as tuberculosis, malaria or typhoid) also can cause chronic fevers. Other sources of chronic fever include:

• Autoimmune diseases such as juvenile rheumatoid arthritis or lupus.
  
  
• Endocrine gland disorders such as hyperthyroidism (excessive secretion of thyroid hormone).
  
  
• Hereditary disorders such as Mediterranean fever (rare disorder that causes fever and pain in the abdomen, chest and joints).
  
  
• Cancers. Malignant tumors and forms of kidney cancer can cause recurring fevers.

In rare cases, no cause can be determined for a persistent fever. Such “fevers of unknown origin” tend to be at least 100.5 degrees Fahrenheit (38.1 degrees Celsius) and last for two to three weeks. In most cases, a physician eventually will find the source of the fever. In younger children, this tends to be viral or bacterial infections. In older children, an inflammation of connective tissue is often responsible.

In addition, recent findings indicate that using positron emission tomography (PET scan) with the radioactive tracer 18-fluoro-deoxy-glucose (18-FDG) can provide early diagnosis of causes of fevers of unknown origin (e.g., cancer) and eliminate the need for additional thorough and invasive diagnostic tests. A PET scan is a minimally invasive radionuclide imaging test that produces three-dimensional images of the living heart, brain, liver or other organs at work. The scan works by creating computer images of the chemical changes that take place inside tissue. These images can provide information on blood flow, oxygen consumption, blood sugar metabolism and concentrations of various compounds in selected organs.

Although the amount of radiation used in PET scans is so low that it poses no health risks to most patients, infants and young children may have a higher level of vulnerability to the radiation.