Image
American policeman wearing a mask to protect himself from Spanish flu, ca. 1918.
Despite gaps in the historical record, it is believed that flu pandemics have occurred throughout human history, when especially nasty strains of influenza virus spread far and wide, sometimes affecting humanity on a global scale.

In the past 100 years, four flu pandemics have spread across the earth. It is estimated that the worst of these pandemics killed 50 million people, and the other outbreaks, while less serious, have still left millions dead.

While flu season appears to be winding down this year, it remains a serious disease that has wreaked havoc on humanity in the past century. Here is a look at each of those pandemics.

Image
© en.wikipedia.org
1918 - 1919, "The Spanish Flu"

According to flu.gov, the 1918 strain of the flu infected between 20 and 40 percent of the worldwide population and killed approximately 50 million people - with nearly 675,000 dying in the United States alone. The pandemic was also notable for disproportionately striking down the young and healthy - as opposed to other flu strains, which are typically most dangerous to the very old and very young.

The disease was so exceptionally deadly that there was originally, according to the CDC, debate that the disease was even the flu at all, and the disease struck in three "unprecedented" waves in 1918 and 1919, with only brief intervals between them.

Image
Members of the Red Guards in China, their mouths masked against flu germs on the orders of Chairman Mao, 1967.
1957 - 1958, "Asian Flu"

Identified in February 1957 in Asia, this pandemic spread "to the United States quietly with a series of small outbreaks," according to flu.gov. According to New Scientist, approximately 2 million people would die from the disease, including nearly 70,000 Americans, with the elderly suffering the highest rate of death.

New Scientist reports that this strain was "triggered by the hybridization" of a human flu virus with "flu viruses from birds."
Image
Undated hand out photo shows the H5N1 or "bird flu" virus as seen through a microscope.
1968-1969, "Hong Kong Flu"

This strain came to the U.S. after being identified in Hong Kong in early 1968, according to flu.gov. Though considered less severe than previous pandemics, this strain reportedly left about a million dead worldwide, and over 33,000 dead in the United States. Several reasons have been given for its relative lack of severity - including better medical care and the idea that it may have been similar to the 1957 strain, which granted people some immunity to the virus.

2009-2010, H1N1

The most recent pandemic on the list, 2009's H1N1 was identified in Mexico, and quickly spread to all 50 states and would, according to health.com, eventually be found in "nearly every country in the world." The strain, also called swine flu as it could circulate among pigs as well as humans, was met with an aggressive vaccination push.

There is a wide range in figures related to the number of deaths from H1N1. The World Health Organization has put the number of confirmed deaths at about 18,000, but a study has suggested that, because deaths from the virus largely occurred in places "where access to prevention and treatment" is limited, the actual number of deaths was underreported and was in reality between 151,700 and 575,400.