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Medical Author:

Mary Nettleman, MD, MS, MACP

Mary Nettleman, MD, MS, MACP

Mary D. Nettleman, MD, MS, MACP is the Chair of the Department of Medicine at Michigan State University. She is a graduate of Vanderbilt Medical School, and completed her residency in Internal Medicine and a fellowship in Infectious Diseases at Indiana University.

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Medical Editor:

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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Swine Flu Cause

Influenza viruses are named according to the types of proteins on the outer surface of the virus. The two main proteins are hemagglutinin (H) and neuraminidase (N). The swine influenza virus in the 2009 outbreak is an H1N1 virus. In fact, although the term swine flu is often used to describe the outbreak, the official term for the virus is novel H1N1 influenza.

It is important to realize that the influenza virus changes (mutates) constantly so that there are many strains of H1N1 that differ subtly from each other. Swine flu is caused by one strain of H1N1, but there are many other strains. Some H1N1 strains only infect pigs. Others infect humans, pigs, and birds. These subtle differences matter because the human body makes antibodies that are tailored to a single strain of influenza. If a person recovers from novel H1N1 (swine) flu, they are probably protected against infection from the same swine flu strain but are not protected against infections from variations on the strain or from other strains of influenza.

The 2009 novel H1N1 swine influenza strain appears to be a result of genetic "reassortment," meaning that it contains pieces of influenza from many different sources. The 2009 virus includes genes that come from bird influenza viruses, swine influenza viruses, and human influenza viruses. This strain has not previously caused infections in humans or pigs. Thus, it is unlikely that humans will have preexisting immunity to this new strain.