Bird flu (avian influenza) is a disease caused by strains of influenza virus that primarily affects birds. In the late 1990s, a new strain of bird flu arose that was remarkable for its ability to cause severe disease and death, especially in domesticated birds such as ducks, chickens, or turkeys. As a result, this strain was called highly pathogenic (meaning very severe and contagious) avian influenza and termed H5N1. A new strain of bird flu was identified in China in 2013. The influenza A virus is termed H7N9 (H7N9 Chinese bird flu). The identification of the virus (H7N9) was reported Mar. 31, 2013; the strain is antigenically different from the H5N1 bird flu virus. Unfortunately, the H7N9 strain of bird flu seems to be genetically unstable. Since its discovery, at least 48 different subtypes of H7N9 have been identified. Because some H7N9 viruses are persistent in some chicken flocks in China, researchers are concerned that the strains will continue to swap genes with other flu viruses and may start a new pandemic. Since the identification of highly pathogenic influenza, infected birds have been found in Asia, Europe, the Middle East, and Africa. Careful control measures, including destroying infected flocks and vaccinating healthy birds, have reduced the number of cases, but the virus continues to exist in poultry flocks in areas of Asia and Africa. In 2007-2008, there were small outbreaks in Bangladesh and Pakistan due to infected domestic chicken flocks. In March 2015 in Arkansas and Missouri, the U.S. Department of Agriculture detected bird flu (H5N2) in several flocks of turkeys, including flocks used to produce the popular Butterball turkeys. This resulted in a number of countries banning U.S. poultry products and had a significant effect on U.S. poultry exports at the time. Similarly, the Dutch poultry industry was affected by the spread of bird flu in chickens in March 2015. Most experts agree that commercial poultry became infected with bird flu due to contamination by wild bird feces. As of March 2015, no bird flu infections have been reported in humans in the U.S. Although the H1N1 "swine flu" pandemic strain contained some bird flu genes, it was not the same strain as the original H5N1 bird flu. The virus spreads through infected birds shedding the virus in their saliva, nasal secretions, and droppings. Healthy birds get infected when they come into contact with contaminated secretions or feces from infected birds. Contact with contaminated surfaces such as cages might also allow the virus to transfer from bird to bird. Symptoms in birds range from mild drops in egg production to failure of multiple major organs and death. The history of bird flu in humans is short. The first human case of illness from highly pathogenic avian influenza (termed HPAI in older literature) was identified in 1997. Since that time, H5N1 has infected 784 individuals with 429 deaths. Human cases of highly pathogenic bird flu have been largely confined to Southeast Asia (India, Bangladesh) and Africa. Mutations often occur in the virus, and it is possible that some mutations could create a more contagious virus that could cause a regional epidemic or a worldwide pandemic of bird flu among humans. Fortunately, the mutations that have occurred to date have not made the virus more contagious, although the concern remains. The identification of H7N9 bird flu strain is worrisome. However, four people in China (two in Shanghai, one in Nanjing, and one in Anhui province) have been identified as being infected with H7N9; two have died. Health officials worldwide are concerned because of the possibility of a bird flu type that could develop easy transmission from birds to humans. Although easy transmission between humans has not yet developed, H7N9 avian influenza virus has infected 622 individuals; 227 of these individuals died from infection to date. The majority of these infections were due to exposure to infected birds or their droppings. In January 2016, an outbreak of H7N8 was reported in turkeys in Indiana; eventually, nine to 10 farms became infected, and thousands of turkeys were killed and the outbreak halted. There were no reports of human infection.