The Spanish flu came on quickly. Some people felt fine in the morning but died by nightfall. People who caught the Spanish Flu but did not die from it often died from complications caused by bacteria, such as pneumonia.
Unlike earlier pandemics and seasonal flu outbreaks, the 1918 pandemic flu saw high mortality rates among healthy adults. In fact, the illness and mortality rates were highest among adults 20 to 50 years old. The reasons for this remain unknown.
1957 – 1958
In February 1957, a new flu virus was identified in the Far East. Immunity to this strain was rare in people younger than 65. Vaccine production began in late May 1957 and was available in limited supply by August 1957.
In the summer of 1957, the virus came to the United States quietly with a series of small outbreaks. When children returned to school in the fall, they spread the disease in classrooms and brought it home to their families.
Most influenza–and pneumonia–related deaths occurred between September 1957 and March 1958. Although the 1957 pandemic was not as devastating as the 1918 pandemic, about 69,800 people in the United States died. The elderly had the highest rates of death.
1968 – 1969
In early 1968, a new flu virus was detected in Hong Kong. The first cases in the United States were detected as early as September 1968 and the virus spread quickly. Deaths from this virus peaked in December 1968 and January 1969.
The number of deaths between September 1968 and March 1969 was 33,800, making it the mildest flu pandemic in the 20th century.
2009 – 2010
In the spring of 2009, a new flu virus spread quickly across the United States and the world. The first U.S. case of HINI (swine flu) was diagnosed on April 15, 2009.
By April 21, the Centers for Disease Control and Prevention (CDC) was working to develop a vaccine for this new virus. On April 26, the U.S. government declared H1N1 a public health emergency.
80 million people were ultimately vaccinated against H1N1, which minimized the impact of the illness. The CDC estimates that 43 million to 89 million people had H1N1 between April 2009 and April 2010. They estimate between 8,870 and 18,300 H1N1 related deaths.