Here's a look at what you need to know about 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."
In February 2013, a strain of bird flu previously not seen in humans - H7N9 - was detected in China.
People have killed hundreds of millions of birds around the world in an attempt to control the spread of the avian flu that has now reached three continents: Asia, Europe and Africa.
There are many different strains of avian flu: 16 H subtypes and 9 N subtypes. Only those labeled H5 or H7 have caused deaths in humans.
Diagnosis/Treatment: 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 percent for infected humans.
Confirmed human cases of Avian Flu (H5N1 strain) Since Dec. 26, 2003: Azerbaijan - 8 cases, 5 deaths Bangladesh - 7 cases, 1 death Cambodia - 38 cases, 29 deaths China - 45 cases, 30 deaths Djibouti - 1 cases, 0 deaths Egypt - 173 cases, 63 deaths Indonesia - 193 cases, 161 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 - 125 cases, 62 deaths Total - 637 cases, 378 deaths
Timeline: 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 - 18 people are infected by the H5N1 strain in Hong Kong, and six die. These are the first documented cases of human infection. The 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 - 84 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 7, 2004 - 12,000 chickens are slaughtered in Kent County, Delaware, after they are found to be infected with the H7 virus.
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.