Discovery Health :: Cold & Flu Center

Symptom Checker

*Back to Ultimate Guide: The Flu main page

Flu's symptoms, from body aches to cough and fever, are legendary. But what causes all that misery? Look below to find out why you feel so awful.

Fever
The infecting viruses trigger the release of pyrogens into the bloodstream. These chemicals stimulate the cold-sensing neurons in the hypothalamus, the body's thermostat. The hypothalamus then raises the body's temperature, stimulating the activity of infection-fighting white blood cells and inhibiting the growth of the temperature-sensitive invading viruses.

Headache
As the body's inflammatory response to the virus kicks in, the arteries in the head expand, irritating nearby nerve endings in the confined space of the head. As the fever increases, the heart rate rises, sending more blood rushing through the body, including the head, resulting in a pounding headache.

Sore Eyes
The mucous membranes of the eyes and nose are connected, and both become inflamed during the infection. The blood vessels in the eye typically become dilated, causing redness, burning and sensitivity to light.

Nasal Congestion
Nasal congestion results when the blood vessels in the mucous membranes expand, engorging the region and leaking fluids and virus-fighting white blood cells into the region in response to the presence of the virus. The result: a stuff, runny nose.

Cough
Inflammation of the respiratory tract and drainage of nasal mucus irritate the throat, triggering the cough reflex. Such a "dry" cough, which doesn't' bring up phlegm, is common in influenza. A "productive" cough may be a sign of a secondary infection such as bronchitis or pneumonia.

Sore Throat
Sore throat and difficulty swallowing are caused by the inflammation of the tissues in your throat as the virus attacks the upper respiratory system.

Fatigue
Fatigue from the flue is caused both by the body devoting energy to the immune response to the virus and the effect of immune-response messenger cells on the nervous system throughout the body.

Body Aches
As the virus spreads, your immune system's white blood cells fight from dominance, and the immune-response messengers that spread throughout the body affect muscle cells, causing muscle aches.

Pictures: Picture(s): PhotoDisc/Getty Images(6) | iStockphoto | PhotoDisc/Getty Images |

SUBSCRIBE TO OUR NEWSLETTERS

HowStuffWorks | TreeHugger | Petfinder | PetVideo | Discovery Education

Visit the Discovery Store: Toys & Games | Telescopes | DVD Sets | Planet Earth DVD | Gift Ideas

By visiting this site, you agree to the terms and conditions
of our Visitor Agreement. Please read. Privacy Policy.
ATTENTION! We recently updated our privacy policy. The changes are effective as of September 10, 2008.
To see the new policy, click here. Questions? See the policy for the contact information.

Copyright © 2009 Discovery Communications, LLC.

The leading global real-world media and entertainment company.