The Spanish influenza pandemic was an unprecedented catastrophe. It is estimated that 20 to 40 percent of the worldwide human population became ill and that more than 50 million people died. In the United States, approximately 675,000 deaths from this pandemic occurred between September 1918 and April 1919. People died very quickly. Some people who felt well in the morning became sick by noon and were dead by nightfall. Those who did not succumb to the disease within the first few days often died of complications such as pneumonia.
One of the most unusual aspects of the Spanish flu was its ability to kill young adults. The reason for this remains uncertain. Mortality rates were high among healthy adults, as well as among the high-risk groups (children and the elderly). Mortality was highest among adults 20 to 50 years old. For more information on Spanish Flu, visit the Centers for Disease Control and Prevention website.
Asian influenza was first identified in the Far East in February 1957. Approximately 69,800 people in the United States died during this pandemic. Since immunity to this strain was rare in people younger than 65 years old, a pandemic had been predicted. In preparation, vaccine production began in late May 1957, and health officials increased surveillance for flu outbreaks. Due to advances in scientific technology, the virus was quickly identified and a vaccine was made available in limited quantities by August 1957.
The United States experienced a series of small outbreaks during the summer of 1957. When children went back to school in the fall, they spread the disease in classrooms and brought it home to their families. Infection rates were highest among school children, young adults, and pregnant women in October 1957. Most influenza and pneumonia-related deaths occurred between September 1957 and March 1958, with the highest death rates occurring in the elderly.
By December 1957, the worst seemed to be over. However, during January and February 1958, there was a second wave of illness among the elderly. A pandemic often has a second wave; as the disease infects one group of people first, infections appear to decrease and then infections increase in a different part of the population.
The Hong Kong influenza pandemic began in early 1968. The first cases in the United States were discovered in September, but illness did not become widespread until December. 33,800 people died between September 1968 and March 1969, many of whom were more than 65 years old. The same virus recurred in 1970 and 1972.
Perhaps fewer people in the United States died during the outbreak because earlier infections caused by the Asian flu virus might have provided some immunity against the Hong Kong flu virus. In addition, instead of peaking in September or October, this pandemic did not gain momentum until near the December school holidays. Since children were at home and did not infect one another at school, the rate of influenza illness among schoolchildren and their families declined. Finally, better medical care and improved antibiotics for secondary bacterial infections were readily available for those who became ill.
Adapted from http://pandemicflu.gov