Negatively-stained transmission electron micrograph (TEM) revealed the presence of a number of Hong Kong flu virus virions, the H3N2 subtype of the influenza A virus. This virus is a Orthomyxoviridae virus family member, and was responsible for the flu pandemic of 1968-1969, which infected an estimated 50,000,000 people in the United States, killing 33,000. Note the proteinaceous coat, or capsid, surrounding each virion, and the hemagglutinin-neuraminidase spikes, which differ in terms of their molecular make-up from strain to strain. There are many different subtypes of type A influenza viruses. These subtypes differ because of changes in certain proteins on the surface of the influenza A virus (hemagglutinin (HA) and neuraminidase (N) proteins). There are 16 known HA subtypes and 9 known NA subtypes of influenza A viruses. Many different combinations of HA and NA proteins are possible. Each combination represents a different subtype. All known subtypes of influenza A viruses can be found in birds.