PANDEMICS OF THE 20TH CENTURY
During the course of the twentieth century, there were three major pandemics. As you already know, the infamous Spanish Flu of 1918 was the first major pandemic. Two smaller flu pandemics also took place: the Asian flu of 1957 and the Hong Kong Flu of 1968.
The Asian Flu (H2N2, 1957)
The Asian Flu of 1957 (H2N2) first appeared in the Southeastern Chinese province of Yunnan. It spread across overland routes to Europe and Africa and sea routes to the Americas, infecting millions of people along the way. Unlike the 1918 strain, people infected with H2N2 did not have the same symptoms that characterized the Spanish Flu of 1918, like viral pneumonia, cyanosis, and acute respiratory distress. In addition, new antibiotics allowed doctors to treat various infections in patients. Over 1 million people died worldwide, including 80,000 Americans. Later examination revealed that this flu strain contained three avian flu genes (PB1, HA, and NA) and five genes from human influenza. Experts believe that antigenic shift was the culprit behind H2N2 and that a pig was the likely mixing vessel.
The Hong Kong Flu (H3N2, 1968)
11 years after the end of the Asian Flu pandemic ended, a new outbreak of avian influenza erupted in Hong Kong. Thought to have originated in the neighboring province of Guangdong, the H3N2 strain was very contagious (there were 500,000 cases in Hong Kong alone). The virus' spread was accelerated by the dawn of air travel; as a result, it spread around the world in less than a year (342 days). The Hong Kong Flu was unusually mild; experts believe that this was because many people had developed antibodies to the H2N2 strain. Because H3N2 had the same neuraminidase molecules, people were immune to the virus. About 750,000 people around the world died of the virus, 34,000 in the United States.