The discovery and spread of a novel coronavirus disease in 2019 and 2020 (COVID-19) has led to a plethora of comparisons to the deadly pandemic that occurred a century earlier—the 1918 influenza pandemic, known colloquially as the “Spanish flu.”

Here is what you should know about the 1918 pandemic that became one of the largest public health crises in modern history.

1. This 1918 influenza pandemic, caused by the Influenza A virus subtype H1N1, produced the greatest influenza (flu) death toll in recorded history. The number of deaths was estimated to be at least 50 million worldwide with about 675,000 occurring in the United States (in comparison, the total number of deaths of World War I was around 20 million). From 1917 to 1920, the virus infected one-third of the Earth’s population, which at the time was about 500 million people. If the same ratio of infections were to happen today, it would be the equivalent of 2.5 billion. That would roughly be the equivalent today of every man, woman, and child in Africa, Europe, and North America becoming infected.

2. The pandemic was commonly known in the United States and Europe as the “Spanish Flu” or the “Spanish Lady.” This was a misnomer, though, as the disease is unlikely to have originated in Spain. The nickname was the result of a widespread misunderstanding caused by wartime news blackouts. To avoid affecting morale, both Allied and Central Powers nations suppressed news about the flu. But because Spain remained neutral during World War I, the Spanish media was free to cover the story. “Since nations undergoing a media blackout could only read in depth accounts from Spanish news sources, they naturally assumed that the country was the pandemic’s ground zero,” says Evan Andrews of “The Spanish, meanwhile, believed the virus had spread to them from France, so they took to calling it the ‘French Flu.’”

3. While the sources of the flu is still unknown (avian and swine origins have been proposed), the first outbreaks appeared in the United States. On March 4, 1918, a U.S. Army private reported to the hospital at Fort Riley, Kansas, complaining of sore throat, fever, and headache. By noon, more than 100 of his fellow soldiers had reported similar symptoms. Other outbreaks soon appeared in Army camps and prisons in various regions of the country. The disease soon spread to Europe with the American soldiers traveling to the battlefields of France. (In the two months after the outbreak at Fort Riley, 202,000 U.S. troops traveled by ship to Europe.)

4. The pandemic occurred in several waves that spread across the globe. The first wave occurred in North America from March through May 1918, and from May through July 1918 in Europe. The second wave—which caused the greatest number of deaths—began in August 1918 and spread across the globe over the next five months. By the end of summer 1918, numerous cases had been reported in China, India, New Zealand, Japan, North Africa, the Philippines, and Russia. A third pandemic wave began in early 1919, just 10 months after the first wave. Some historians also claim that a fourth wave occurred in early 1920.

5. The pandemic was exacerbated by poor sanitation, overcrowding, and limited health services during World War I. Many U.S. soldiers with immune systems that has never been exposed to the flu were crowded into hastily built camps and ships. Each day in summer 1918, an average of 10,000 U.S. soldiers crammed onto ships bound for France, and 45,000 soldiers were corralled into camps built to accommodate 36,000. As a result, in 1918 more American troops died from flu than they did on the battlefield.

6. According to the Centers for Disease Control, an unusual characteristic of this virus was the high death rate it caused among healthy adults 15 to 34 years of age. The pandemic lowered the average life expectancy in the United States by more than 12 years. The estimated case fatality rate was 1.7 percent. A comparable death rate has not been observed during any of the known flu seasons or pandemics that have occurred either prior to or following the 1918 pandemic. (The death rate from seasonal flu is typically around 0.1 percent in the United States.) The rate of death was likely higher because, at the time, there were no flu vaccines, antiviral drugs, antibiotics, or mechanical ventilators. About one-third of doctors and nurses from the United States were also serving in the war, making treatment in the United States less available.

7. Because there was no coordinated effort by the U.S. government to implement mitigation efforts, local communities implemented their own measures. For instance, the health commission of New York City attempted to slow the transmission of the flu by ordering businesses to open and close on staggered shifts to avoid overcrowding on the subways. At the time, 43 U.S. cities had a population of more than 100,000. Cities that implemented measures such as school closures, bans on public gatherings, and isolation or quarantine orders experienced delayed and reduced peak death rates compared with cities that implemented interventions later.

8. During the pandemic, restrictions on public gatherings affected churches. In Washington, D.C., a group of Protestant ministers “voted unanimously to accede to the request of the District Commissioners that churches be closed in the city.” Churches were also closed in cities such as Dallas, Milwaukee, Los Angeles, and Seattle, yet remained open in Chicago and San Francisco. But, much like today, such measures weren’t always popular. A Baptist pastor in Murray, Kentucky, held services on January 26, 1919 in violation of the state’s ban and was arrested in his pulpit at the evening service. A Catholic priest in St. Louis was allegedly turned in to police after 200 parishioners were seen at the church. The priest told police the people snuck in through the church’s side windows ,and he didn’t see them. No charges were pressed.

9. Since 1918, there have been several other influenza pandemics. A flu pandemic from 1957 to 1958 killed around 2 million people worldwide, including some 70,000 people in the United States, and a pandemic from 1968 to 1969 killed approximately 1 million people, including some 34,000 Americans. More than 12,000 Americans perished during the H1N1 (or “swine flu”) pandemic that occurred from 2009 to 2010. But the 1918 influenza pandemic has remained not only the deadliest flu of the modern age, but also one of the most lethal virus-borne diseases, killing more people than all subsequent flu pandemics, yellow fever (late 1800s), the Cholera 6 outbreak (1817-1923), SARS (2002-2003), Ebola (2014-2016), and HIV/AIDS (1981–present) combined.

Addendum: While there are several similarities between the 1918 flu pandemic and COVID-19 (short for novel coronavirus 2019), they are different in numerous and significant ways. COVID-19 is not a strain of flu, but rather a disease caused by a strain of coronavirus (SARS-CoV-2). What we call “flu” is several different types and strains of influenza viruses. Coronaviruses are a different and distinct type of virus.

The key reason COVID-19 is currently more dangerous than common strains of flu is because we have no protection against it (NB: the 1918 flu was an uncommon strain). For all of us alive today, strains of the flu have existed our entire life. Almost everyone gets the flu at some stage of life and are therefore able to build up immunity. We also have vaccines created each year that protect people from new strains. Despite these immunities, we still have 291,000 to 646,000 deaths worldwide from the flu each year.

SARS-CoV-2 is a new strain of coronavirus for which we haven’t built up an immunity. Also, unlike many influenza strains, we have no vaccines to protect us against the virus. Additionally, SARS-CoV-2 is believed to be about seven- to ten-times more lethal than the average strain of influenza. Because of these factors, COVID-19 has the potential to kill many more times as many people as are killed by the annual strains of flu. The COVID-19 infections also have to be treated in addition to the hospitalizations and deaths that occur from influenza. Our health-care system already becomes strained each year during flu season, so adding tens or hundreds of thousands of COVID-19 cases at one time will stretch our system to the breaking point (as it has already done in Italy).