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Flu Symptom Checker

The flu is no picnic.
The flu is no picnic.
Peter Dazeley/Photographer's Choice/Getty Images

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.


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.


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.


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 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.