(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.
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.
The most commonly seen and most deadly form of the virus is called "Influenza A (H5N1)," or the "H5N1 virus."
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 or feces.
Symptoms of avian flu include fever, cough, sore throat and sometimes severe respiratory diseases and pneumonia.
The CDC recommends oral oseltamivir (brand name: Tamiflu), inhaled zanamivir (brand name: Relenza) and intravenous permavir (brand name: Rapivab) for the treatment of human illness associated with avian flu.
The mortality rate is close to 60% for infected humans.
Early 1900s -The avian flu is first identified in Italy.
1961 - The H5N1 strain is 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.
1997 - Eighteen people are infected by the H5N1 strain in Hong Kong, six die. These are the first documented cases of human infection. Hong Kong destroys its entire poultry population, 1.5 million birds.
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, one dies.
February 7, 2004 - Twelve thousand chickens are killed in Kent County, Delaware, after they are found to be infected with the H7 virus.
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 H5N1 strain of avian influenza.
November 16, 2005 - The World Health Organization 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 die.
January 17, 2006 - A 15-year-old girl from northern Iraq dies after contracting bird 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.
May 11, 2006 - Djibouti announces its first cases of H5N1 - several birds and one human.
December 20, 2011 - The US 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 31, 2013 - Chinese authorities report the first human cases of infection of avian flu H7N9 to the World Health Organization. H7N9 has not previously been detected in humans.
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.
April 20, 2015 - Officials say more than five million hens will be euthanized after bird flu was detected at a commercial laying facility in northwest Iowa. According to the US Department of Agriculture, close to eight 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.
January 15, 2016 - The US Department of Agriculture confirms that a commercial turkey farm in Dubois County, Indiana, has tested positive for the H7N8 strain of avian influenza.
January 24, 2017 - Britain's Department for Environment, Food & Rural Affairs releases a statement confirming that a case of H5N8 avian flu has been detected in a flock of farmed breeding pheasants in Preston, UK. The flock is estimated to contain around 10,000 birds. The statement adds that a number of those birds have died, and the remaining live birds at the premises are being "humanely" killed because of the disease.
February 12, 2017 - A number of provinces in China have shut down their live poultry markets to prevent the spread of avian flu after a surge in the number of infections from the H7N9 strain. At least six provinces have reported human cases of H7N9 influenza this year, according to Chinese state media, Xinhua.
March 5-7, 2017 - The USDA confirms that a commercial chicken farm in Tennessee has tested positive for the H7N9 strain of avian flu, but says it is genetically different from the H7N9 lineage out of China. The 73,500-bird flock in Lincoln County will be euthanized, according to Tyson Foods.
February 14, 2018 - Hong Kong's Centre for Health Protection announces that a 68-year-old woman has been treated for the H7N4 strain. This is the first case of this strain in a human.
June 5, 2019 - Since 2013 there have been 1,568 confirmed human cases and 616 deaths worldwide from the H7N9 strain of avian flu, according to the Food and Agriculture Organization of the United Nations.
December 2019 - The United Kingdom Department for Environment, Food & Rural Affairs confirms that a case of H5N1 avian flu has been detected at a poultry farm in Suffolk. 27,000 birds are humanely killed because of the disease.
April 9, 2020 - The USDA confirms that a commercial turkey flock in Chesterfield County, South Carolina has tested positive for the H7N3 strain of avian flu.
January 2021 - India culls tens of thousands of poultry birds after avian influenza is detected in ducks, crows and wild geese in at least a dozen locations across the country.
February 18, 2021 - Russian authorities notify WHO that they have detected H5N8 in humans. "If confirmed, this would be the first time H5N8 has infected people," a WHO Europe spokesperson says in a statement.
June 1, 2021 - China's National Health Commission announces the first human case of H10N3.