health

Bacteria and viruses themselves, as well as toxins (poisonous waste products)
produced by some bacteria, cause fever. In some cases, they work
directly on the brain to raise the thermostat. More commonly, they cause
the body’s immune system to produce proteins called cytokines (SY-tokines).
The cytokines help fight the infection, but they also reset the
brain’s thermostat, causing fever.
Any substance that causes fever is called a pyrogen (PY-ro-jen), from
the Greek word for “fire-causer.” If the substance comes from outside the
body, such as a toxin from bacteria, it is called an exogenous (ek-SOJ-enus)
pyrogen. The prefix “exo-” means “outside” in Greek. If the substance
comes from inside the body, such as a cytokine, it is called an endogenous
(en-DOJ-e-nus) pyrogen. The prefix “endo-” means “inside” in Greek.

Sometimes the immune system produces pyrogens even without an
infection. For instance, this may happen if a person has the following:

• an autoimmune disease, such as rheumatoid arthritis or lupus
  
• inflammation anywhere in the body
  
• cancer, such as leukemia or lymphoma
  
• received a blood transfusion* that is not compatible with his or her
  own blood type
  
• has a reaction to a medication