The H3N2 subtype of the influenza A virus infects birds, pigs and humans, causing infectious seasonal influenza. In a pandemic in 1968 known as the "Hong Kong Flu," a H3N2 variant infected an estimated 500,000 people in Hong Kong. Influenza viruses such as H3N2 are RNA viruses, with a single stranded genome encapsulated within an envelope layer. Hemagglutinin and neuraminidase surface antigens within this envelope are typical targets of H3N2 antibodies.