(CNN)Here's a look at avian flu.
Avian influenza, also called avian flu or bird flu, is an illness that usually affects only birds. A new strain jumped to baby seals in 2011. While there have been deaths from certain strains of avian flu, person-to-person infections are rare.
The official name for the most commonly seen and most deadly form of the virus is called "Influenza A (H5N1)," or the "H5N1 virus."
People have killed hundreds of millions of birds around the world in an attempt to control the spread of the avian flu.
There are many different strains of avian flu: 16 H subtypes and 9 N subtypes. Only those labeled H5, H7 and H10 have caused deaths in humans.
Most cases of human bird flu infections are due to contact with infected poultry or surfaces that are contaminated with infected bird excretions: saliva, nasal secretions and feces.
Symptoms of avian flu include fever, cough, sore throat and sometimes severe respiratory diseases and pneumonia.
The FDA has approved four drugs to combat influenza - amantadine, rimantadine, oseltamivir (brand name: Tamiflu) and zanamivir (brand name: Relenza).
H5N1 is resistant to two of the FDA approved drugs used to combat influenza - amantadine and rimantadine.
The mortality rate is nearly 60% for infected humans.
Confirmed human cases of avian flu (H5N1 strain) since December 26, 2003:
Azerbaijan - 8 cases, 5 deaths
Bangladesh - 7 cases, 1 death
Cambodia - 56 cases, 37 deaths
Canada - 1 case, 1 death
China - 51 cases, 31 deaths
Djibouti - 1 cases, 0 deaths
Egypt - 329 cases, 107 deaths
Indonesia - 199 cases, 167 deaths
Iraq - 3 cases, 2 deaths
Laos - 2 cases, 2 deaths
Myanmar - 1 case, 0 deaths
Nigeria - 1 case, 1 death
Pakistan - 3 cases, 1 death
Thailand - 25 cases, 17 deaths
Turkey - 12 cases, 4 deaths
Vietnam - 127 cases, 64 deaths
Total - 826 cases, 440 deaths
Early 1900s -The avian flu is first identified in Italy.
1961 - The H5N1 strain is first isolated in birds in South Africa.
December 1983 - Chickens in Pennsylvania and Virginia are exposed to the avian flu, and more than five million birds are killed to stop the disease from spreading.
May 1997 - Eighteen people are infected by the H5N1 strain in Hong Kong, and six die. These are the first documented cases of human infection. Hong Kong destroys its entire poultry population (1.5 million birds) in three days.
1999 - Two children in Hong Kong are infected by the H9N2 strain.
February 2003 - Eighty-four people in the Netherlands are affected by the H7N7 strain of the virus, and one dies.
January 25, 2004 - China bans imports of Thai chicken products. Nine million chickens are slaughtered in Thailand in an effort to stop the spread of the disease.
February 2, 2004 - The World Health Organization is investigating possible human-to-human transmission among a family in Vietnam. Three family members have died of the illness, and one has recovered. One member had no contact with infected poultry while the others did.
February 23, 2004 - A flock of 6,600 broiler chickens in Gonzalez County, Texas, is destroyed after being diagnosed with an "extremely infectious and fatal" form of bird flu, the H5N2 strain.
February 5, 2005 - The Cambodian Health Ministry and WHO confirm the first human death in Cambodia (the H1N1 strain, on January 30).
October 7, 2005 - The avian flu reaches Europe. Romanian officials quarantine a village of about 30 people after three dead ducks there test positive for bird flu.
November 12, 2005 - A one-year-old boy in Thailand tests positive for the lethal H5N1 strain of avian influenza.
November 16, 2005 - WHO confirms two human cases of bird flu in China, including a female poultry worker who died from the H5N1 strain.
November 17, 2005 - Two deaths are confirmed in Indonesia from the H5N1 strain of avian influenza.
January 1, 2006 - A Turkish teenager dies of the H5N1 strain of avian influenza in Istanbul, and later that week, two of his sisters follow.
January 17, 2006 - A 15-year-old girl from northern Iraq dies after contracting bird flu.
February 20, 2006 - The French Health Ministry confirms that a duck in central France had the H5N1 strain of avian flu.
February 20, 2006 - Vietnam becomes the first country to successfully contain the disease. A country is considered disease-free when no new cases are reported in 21 days.
March 12, 2006 - Officials in Cameroon confirm cases of the H5N1 strain. The avian flu has now reached four African countries.
March 13, 2006 - The avian flu is confirmed by officials in Myanmar (Burma).
May 11, 2006 - Djibouti announces its first cases of H5N1 - several birds and one human.
December 20, 2011 - The U.S. Department of Health and Human Services releases a statement saying that the government is urging scientific journals to omit details from research they intend to publish on the transfer of H5N1 among mammals. There is concern that the information could be misused by terrorists.
July 31, 2012 - Scientists announce that H3N8, a new strain of avian flu, caused the death of more than 160 baby seals in New England in 2011.
March 2013 - The official Chinese news agency Xinhua reports that two people in China have died after falling ill with a strain of bird flu, H7N9, not detected before in humans.
December 2, 2013 - A woman in Hong Kong is hospitalized with H7N9 after coming in contact with live poultry in the mainland China city of Shenshen.
December 6, 2013 - A 73-year-old woman infected with H10N8 dies in China, the first human fatality from this strain.
January 8, 2014 - Canadian health officials confirm that a resident from Alberta has died from H5N1 avian flu, the first case of the virus in North America. It is also the first case of H5N1 infection ever imported by a traveler into a country where the virus is not present in poultry.
March 3, 2015 - A WHO report indicates that so far there have been 602 laboratory-confirmed cases of H7N9, including 227 deaths.
April 20, 2015 - Officials say more than 5 million hens will be euthanized after bird flu was detected at a commercial laying facility in northwest Iowa. According to the U.S. Department of Agriculture, close to 8 million cases of bird flu have been detected in 13 states since December. Health officials say there is little to no risk for transmission to humans with respect to H5N2. No human infections with the virus have ever been detected.