6 of the Worst Pandemics in History

In late December 2019, the world was introduced to a novel coronavirus—SARS-CoV-2—a pathogen that causes COVID-19. Just two months later, the World Health Organization (WHO) declared the coronavirus outbreak a pandemic, meaning the virus had spread across several countries and sickened a large number of people. By May 2020, COVID-19 was responsible for over 520 million cases worldwide, according to the WHO.

Realistically speaking, the next pandemic wasn't a far-fetched idea—in part, because it's already happened multiple times before: "When we talk about another flu pandemic happening, it's not a matter of if, but when," said Dennis Carroll, PhD, director of USAID's Emerging Threats Unit, in the trailer for the Netflix docuseries, Pandemic: How to Prevent an Outbreak. (FYI: Coronavirus is not influenza, though the symptoms may sometimes look similar.)

Our extremely interconnected world is largely to blame. "Viruses used to spread at the speed of a steamboat; now they can spread at the speed of a jet. In that sense, we're more at risk," infectious disease expert Amesh A. Adalja, MD, senior scholar at the Johns Hopkins Center for Health Security, told Health. There are also more densely populated cities now than ever before, Dr. Adalja pointed out, and that can create an environment that's ripe for the rapid spread of pandemic diseases. On the flip side, there's a "very sophisticated" healthcare system in place, both in the U.S. and globally, said Dr. Adalja. "We have antivirals, vaccines, and pharmaceutical companies that can develop countermeasures for new emerging diseases," he said.

In talking about future pandemics, the Netflix series also dives briefly into a few of the worst flu pandemics of the past, particularly the Spanish flu pandemic of 1918. Here's a look at some of the worst pandemics—both flu-related and not, though excluding the current coronavirus pandemic—in modern history.

The Great Plague of London

The bubonic plague technically made its first appearance in pandemic levels in the 14^th century, with The Black Death, but it surfaced for a second time in London in 1665 for the Great Plague of London, which killed 20% of London's population, according to History.com. The death toll was so high that mass graves appeared, and thousands of cats and dogs, who were believed to be the cause of the source, were slaughtered. The outbreak eventually tapered off in 1666.

Surprisingly, the bubonic plague is still around today—sometimes occurring in rural areas in the Western United States, though it's more common in parts of Africa and Asia, according to the Centers for Disease Control and Prevention. Humans typically contract the plague—which is broken down into the bubonic plague, septicemic plague, and pneumonic plague, depending on transmission—through contact with an infected flea or animal. Symptoms of the bubonic plague, in particular, include a sudden onset of fever, headache, chills, weakness, and one or more swollen, tender and painful lymph nodes. Luckily, per the CDC, modern-day antibiotics are successful in treating the plague.

The Spanish Flu

The Spanish flu was an influenza pandemic that spread around the world between 1918 and 1919, according to the CDC. It was caused by an H1N1 virus, with an avian (bird) origin, though it's unclear exactly where the virus originated. The CDC estimates that about 500 million people (or one-third of the world's population) became infected with the virus. It ultimately caused at least 50 million deaths worldwide with about 675,000 deaths happening in the U.S.

The 1918 flu was especially virulent, per the CDC. While much remains undocumented about the Spanish flu, the CDC notes that one well-documented effect was rapid and severe lung damage. "In 1918, victims of the pandemic virus experienced fluid-filled lungs, as well as severe pneumonia and lung tissue inflammation," according to the CDC.

Scientists also worked to replicate the 1918 flu virus, beginning in 2005, to evaluate the virus' pathogenicity, or its ability to cause disease and harm a host. The work, led by Terrence Tumpey, PhD, a microbiologist and chief of the Immunology and Pathogenesis Branch (IPB) of the CDC's Influenza Division, showed that the 1918 influenza virus was a "uniquely deadly product of nature, evolution, and the intermingling of people and animals," per the CDC, and may help with future possible pandemics.

Asian Flu

Another flu pandemic, the "Asian Flu" began in East Asia in 1957, according to the CDC. That specific influenza virus was an H2N2 strain, which was first detected in Singapore in February 1957. From there, the virus made its way to Hong Kong in April 1957, and in coastal cities in the United States in the summer of 1957. An estimated 1.1 million people died of the Asian flu worldwide, with 116,000 of them in the United States.

1968 Flu Pandemic

The flu pandemic of 1968—also called the Hong Kong Flu—originated in China in July 1968, according to Encyclopaedia Britannica. Caused by an influenza A virus (H3N2), it was the third pandemic flu outbreak to occur in the 20th century, killing one million people worldwide and about 100,000 people in the US alone.

According to Encyclopaedia Brittanica, it's believed that 1957's Asian Flu pandemic may have given rise to the 1968 pandemic through a process called "antigenic shift," in which there are small changes to the genes of a flu virus which can lead to changes in the surface proteins of a virus—HA (hemagglutinin) and NA (neuraminidase)—which trigger the body's immune response. Ultimately, those antigenic shifts are why people can get the flu more than once, according to the CDC, and why a yearly flu vaccine is necessary for the best protection against the virus and its ever-changing nature.

2009 Flu Pandemic

The most recent flu pandemic in the US, initially known as "swine flu," occurred in 2009 with a novel influenza virus, H1N1, not previously identified in either animals or humans, per the CDC. The virus was actually first detected in the US and spread quickly across the US and the world. According to the CDC, between April 12, 2009, and April 10, 2010, there were 60.8 million cases, 274,304 hospitalizations, and 12,469 deaths (range: 8868-18,306) in the US due to the virus. The CDC also estimated that up to 575,400 people died worldwide.

According to the CDC, the 2009 flu pandemic primarily affected children and middle-aged adults (older adults had immunity, likely from previous exposure to a similar H1N1 virus). And while the pandemic officially ended on August 10, 2010, the (H1N1)pdm09 virus continues to circulate as a seasonal flu virus, causing illness, hospitalization, and deaths worldwide every year.

HIV/AIDS

Human immunodeficiency virus (HIV) and acquired immunodeficiency syndrome (AIDS) were first discovered in the early 1980s. AIDS was first detected in American gay communities, but it's thought to have developed from a chimpanzee virus from Africa in the 1920s. According to the WHO, 79.3 million people have been infected with the HIV virus since the beginning of the epidemic, and 36.3 million people have died of HIV. Globally, 37.7 million people were living with HIV at the end of 2020. New treatments, however, have allowed more people to live with HIV, and about 1.1 million Americans currently have the disease, according to government data. An estimated 38,000 new HIV infections still happen in the US each year.