H5N1 is a type of avian influenza virus (bird flu virus) that has mutated[1] through antigenic drift into dozens of highly pathogenic varieties, but all currently belonging to genotype Z of avian influenza virus H5N1. Genotype Z emerged through reassortment in 2002 from earlier highly pathogenic genotypes of H5N1[2] that first appeared in China in 1996 in birds and in Hong Kong in 1997 in humans[3]. The "H5N1 viruses from human infections and the closely related avian viruses isolated in 2004 and 2005 belong to a single genotype, often referred to as genotype Z." [1]

The avian influenza subtypes that have been confirmed in humans, ordered by the number of known human deaths, are: H1N1 caused Spanish flu, H2N2 caused Asian Flu, H3N2 caused Hong Kong Flu, H5N1, H7N7, H9N2, H7N2, H7N3, H10N7.

All avian influenza (AI) viruses are type A influenza virus in the virus family of Orthomyxoviridae and all known strains of influenza A virus infect birds. Influenzavirus type A is subdivided into subtypes based on hemagglutinin (H) and neuraminidase (N) protein spikes from the central virus core. There are 16 H types, each with up to 9 N subtypes, yielding a potential for 144 different H and N combinations.

Avian influenza (also known as bird flu, avian flu, influenzavirus A flu, type A flu, or genus A flu) is a flu due to a type of influenza virus that is hosted by birds, but may infect several species of mammals.

An influenza pandemic is a large scale epidemic of the influenza virus, such as the 1918 Spanish flu. The World Health Organization (WHO) warns that there is a substantial risk of an influenza pandemic within the next few years. One of the strongest candidates is the A(H5N1) subtype of avian influenza.

A myxovirus of the genus Influenzavirus, antigenically varying from influenza virus type A and influenza virus type C, that causes various respiratory illnesses in humans.

A myxovirus of the genus Influenzavirus, antigenically varying from influenza virus type B and influenza virus type C, that causes acute respiratory illness in humans.

Influenza caused by infection with a strain of influenza virus type C.

People should get vaccine who are:

People 65 years of age or older. Resident of nursing home and other chronic-care facilities. Adults and adolescents with chronic pulmonary or cardiovascular disorders, including asthma. Health care workers, care givers and others who might transmit influenza virus to persons at high-risk for complications from infection.

People who are less able to fight infections because of a disease they are born with, infection with Human Immunodeficiency Virus (HIV), treatment with medications such as long-term steroids, and/or treatment for cancer with X-rays or medications.

Adults and adolescents who required regular medical follow-up or hospitalization during the preceding year because of chronic illnesses (including diabetes mellitus), kidney diseases, and blood cell diseases such as sickle cell anemia.

Women who will be in the second or third trimester of pregnancy during the flu season (December - March).

Persons 6 months to 18 years of age who receive long-term aspirin therapy and therefore might be at risk for developing Reye syndrome after influenza.