Facebook Twitter Instagram You Tube Pinterest LinkedIn RSS Podcasts Video Library Blog
 - Northwestern Memorial Hospital - Chicago


What is influenza, or “flu”?

According to the National Institute of Allergy and Infectious Diseases, influenza is a contagious respiratory infection that is caused by a variety of human influenza viruses. These viruses change from year to year which is why it is recommended to get the vaccine to protect against infection every year. The “stomach flu” is not caused by the influenza virus but is a stomach or intestinal infection caused by other viruses, bacteria, or parasites.

What is the difference between a cold and the flu?

In general, the flu is worse than the common cold, and the symptoms are more intense. In addition to having a sore throat, cough, and possibly a runny nose, you may have the flu if you have a rapid onset of a high fever (usually above 102°), severe aches and pains, and a bad headache. For a complete comparison between colds and flu, go to http://www.cdc.gov/flu/about/qa/coldflu.htm.

The human influenza viruses differ from the avian (bird) influenza viruses, which usually do not infect humans. Only humans in close contact with domestic poultry have been infected; the avian flu virus has only rarely spread from humans to other humans. http://www.cdc.gov/flu/avianflu/avian-in-humans.htm

The swine influenza virus is common in pig populations in many parts of the world, including the United States. Swine flu viruses do not normally infect humans, however, sporadic human infections with swine flu virus have occurred. It is important to note that swine flu is not caused by consuming pork or pork products. http://www.cdc.gov/flu/swineflu/keyfacts-variant.htm

Internet Resources

MedlinePlus: Influenza
Developed at the National Library of Medicine specifically for health care consumers, this site is a portal for trusted government-sponsored and privately developed health information.

US Dept of Health and Human Services: Flu.gov
This website provides links to U.S. government information on swine, avian, or pandemic flu.

CDC: Influenza
Comprehensive site that includes weekly flu activity reports for the United States and the rest of the world. Pages are available in five languages.


  • Inside the outbreaks: the elite medical detectives of the epidemic intelligence service. Pendergrast M. 2010.
  • Twelve diseases that changed our world. Sherman IW. 2007.
  • China syndrome: the true story of the 21st century’s first great epidemic. Greenfeld KT. 2006.
  • Great influenza: the epic story of the deadliest plague in history. Barry JM. 2004.
  • Flu: the story of the great influenza pandemic of 1918 and the search for the virus that caused it. Kolata GB. 1999.

Journals and Journal Articles

  • “Why is it so important to get a flu shot every year?” Duke Medicine Health News. 17(11):8. Nov 2011.
  • “Flu vaccine. Facts about shots, sprays, and seasonal protection.” Mayo Clinic Women’s Healthsource. 14(10):1–2. Oct 2010.
  • “How do you know whether it’s flu?” Harvard Health Letter. 35(3):3. Jan. 2010.
  • “Colds and influenza: A review of diagnosis and conventional, botanical, and nutritional considerations.” Alternative Medicine Review. March 2007. 12(1):25–48.

Contact Us

For more information, please contact the Health Learning Center at 312.926.5465 or HLC@nmh.org.

Last UpdateMarch 5, 2012