(CNN) -- Here's a look at what you need to know about the H1N1 influenza virus, also known as swine flu. There was a pandemic outbreak across the globe which lasted from 2009 to 2010.
Confirmed human cases of H1N1: April 2009-February 2010 -Total fatalities in the United States are between 8,720 and 18,050.
Fatalities Worldwide - Only 18,500 confirmed deaths, but estimates are 151,700 to 575,400.
Swine Flu: The H1N1 virus is a type of swine flu.
Swine Influenza (swine flu) is a respiratory disease of pigs caused by type A influenza virus. Swine flu viruses cause high levels of illness but low death rates in pigs.
Swine flu is transmitted from contact with pigs or their contaminated food or air. It is transmitted between humans when someone touches an object coughed or sneezed on by an infected person, and then touches his/her mouth or nose. However, it cannot be passed from pork or other meat products to humans.
Swine flu outbreaks in pigs mostly occur during the late fall and winter months.
It is a constantly mutating virus. Pigs are susceptible to viruses from birds, humans and other swine. When these different influenza viruses strike pigs, the genes can mutate and new viruses can develop.
Swine Flu in Humans: Swine flu is not common in humans. Occasionally, human infections with swine flu occur in people who have been exposed to pigs (e.g. children near pigs at a fair or workers in the swine industry).
Symptoms include fever, lethargy, lack of appetite and coughing. Other symptoms include runny nose, sore throat, nausea, vomiting and diarrhea.
Drugs that treat swine flu in humans include oseltamivir (Tamiflu) and zanamivir (Relenza).
Timeline: 1930 - The classical swine flu virus (an influenza type A H1N1 virus) is first isolated from a pig.
1976 - Swine flu (Hsw1N1) breaks out among soldiers at Fort Dix, New Jersey. Thirteen soldiers are infected and one dies.
1976 - The United States begins a nationwide vaccination program against a type of swine flu known as Influenza A/New Jersey/76. However, the program is suspended after people die within hours of receiving the vaccination. More than 500 people develop Guillian-Barre syndrome after the vaccination, and 32 people die.
September 1988 - A woman dies of the H1N1 flu virus days after visiting a county fair pig exhibition where there was widespread influenza-like illness among the swine.
December 2005-February 2009 - The Centers for Disease Control and Prevention reports 12 cases of swine flu among humans.
April 24, 2009 - The CDC issues an outbreak notice warning travelers of an increased health risk of swine flu in Central Mexico and Mexico City.
April 26, 2009 - The United States declares a public health emergency as cases of swine flu increase.
April 27, 2009 - World Health Organization raises the influenza pandemic alert to a level 4.
April 29, 2009 - WHO raises the influenza pandemic alert to a level 5.
June 11, 2009 - WHO raises the influenza pandemic alert to a level 6.
October 24, 2009 - U.S. President Barack Obama declares the H1N1 outbreak a national emergency.
June 25, 2012 - A study published in The Lancet Infectious Disease journal suggests that the death toll from the 2009 pandemic could be 15 times higher than previously reported.