The Hong Kong flu, also known as the 1968 flu pandemic, was a flu pandemic whose outbreak in 1968 and 1969 killed between one and four million people globally. It is among the deadliest pandemics in history, and was caused by an H3N2 strain of the influenza A virus. The virus was descended from H2N2 (which caused the Asian flu pandemic in 1957–1958) through antigenic shift, a genetic process in which genes from multiple subtypes are reassorted to form a new virus. History Origin and outbreak in Hong Kong and China The first recorded instance of the outbreak appeared on 13 July 1968 in British Hong Kong. It has been speculated that the outbreak began in mainland China before it spread to Hong Kong; On 11 July, before the outbreak in the colony was first noted, the Hong Kong newspaper Ming Pao reported an outbreak of respiratory illness in Guangdong Province, and the next day, The Times issued a similar report of an epidemic in southeastern China. Later reporting suggested that the flu had spread from the central provinces of Sichuan, Gansu, Shaanxi, and Shanxi, which had experienced epidemics in the spring. However, due to a lack of etiological information on the outbreak and a strained relationship between Chinese health authorities and those in other countries at the time, it cannot be ascertained whether the Hong Kong virus was to blame. The outbreak in Hong Kong, where the population density was reached its maximum intensity in two weeks. The outbreak lasted around six weeks, affecting about 15% of the population (some 500,000 people infected), but the mortality rate was low and the clinical symptoms were mild. There were two waves of the flu in mainland China, one between July–September in 1968 and the other between June–December in 1970. The reported data were very limited due to the Cultural Revolution, but retrospective analysis of flu activity between 1968 and 1992 shows that flu infection was the most serious in 1968, implying that most areas in China were affected at the time. Outbreaks in other areas Despite the lethality of the 1957–1958 pandemic in China, little improvement had been made regarding the handling of such epidemics. By 13 August, was clear to virologists that strains isolated from the outbreak in Hong Kong differed markedly from previous strains of influenza. However, they were not at the time considered to be an entirely new