Bird flu is caused by strains of the influenza virus that have evolved to be specially adapted to enter avian cells. There are three main types of influenza: A, B, and C. The virus that causes bird flu is influenza A type with eight RNA strands that make up its genome. Influenza viruses are further classified by analyzing two proteins on the surface of the virus. The proteins are called hemagglutinin (H) and neuraminidase (N). There are many different types of hemagglutinin and neuraminidase proteins. For example, the recent pathogenic bird flu virus has type 5 hemagglutinin and type 1 neuraminidase. Thus, it is named "H5N1" influenza A virus (also termed HPAI or highly pathogenic avian influenza). The 2013 virus has different surface proteins, H7 and N9, hence the name H7N9. Other bird flu types include H7N7, H5N8, H5N2, and H9N2. There are many types of influenza viruses, and most prefer to live in a limited number of animals. Thus, swine flu primarily infects swine, and bird flu primarily infects birds. Human influenza strains are best adapted to humans. A few cases may occur in an accidental host, such as when people who have extensive contact with sick birds get the bird flu. In addition to humans and birds, we know pigs, tigers, leopards, ferrets, and domestic cats and dogs can sometimes be infected with avian influenza viruses. Influenza viruses mutate easily and often. These mutations can arise spontaneously in a single virus or can occur when two different influenza strains get close enough together to exchange genetic material. There are two major types of mutations in influenza viruses: antigenic shifts, where large RNA segments are interchanged between different influenza virus type, and antigenic drifts, where small RNA sequences are changed. The antigenic shifts are usually responsible for developing new strains. For example, the 2009 swine flu pandemic was caused by a virus that included genetic material from pig influenza, avian influenza, and human influenza strains. New mutations can allow the virus to evade the body's immune system and makes older vaccines ineffective. In 2011, one strain of the highly pathogenic avian influenza virus mutated in this way, making the existing vaccine used against avian flu ineffective against the new strain. Sometimes a flu virus will mutate in a way that makes it able to infect a new species. Serious pandemic influenza occurs when a relatively new strain of the influenza virus arises that is highly contagious to humans. The most deadly pandemic in modern history was the 1918 influenza, also known as the Spanish flu (although it did not originate in Spain). The 1918 virus spread rapidly and killed tens of millions of people worldwide. Mortality was especially high in healthy young adults. Although the 1918 virus was a human influenza virus, it had many genes that likely came from a strain of bird flu. One reason health officials carefully watch for and try to limit human contact with birds that develop avian flu is to avoid chances for a new strain to arise that may prefer to develop in human tissue.