An infectious disease is any sickness caused by harmful germs getting into the body. When a disease spreads to many people across many parts of the world, it becomes a pandemic. People in many different times and places have faced danger from pandemics. But our understanding of where infectious diseases come from, how they spread, and how we can help people get better has changed A LOT over time.

People can catch infectious diseases from other people, animals, insects, or their environment.

Estimated 30–50 million people died

How would you feel if you had a plague named after you? That’s what happened to Byzantine Emperor Justinian I. During his reign, millions of people caught what is now known as the bubonic plague. The emperor was one of these million, and though he recovered, the outbreak that happened during his rule was forever associated with his name.

Those who caught the plague suffered terrible symptoms, like swelling sores called buboes, fever, vomiting, and blackened fingertips. Worse, most people who caught the plague died. At the time, no one knew what caused the disease. But today, we know that a bacterium carried by fleas causes the bubonic plague. Historians disagree about how big this plague was.

Black fingertips were a common symptom of the plague. It happened when the bacterium got into a person’s bloodstream, which stopped blood from reaching their fingers, toes, or sometimes their nose.

Some think it killed about half of the world’s population at the time and was a major reason the Roman Empire fell.

Other historians argue that the death toll and impact were smaller. Though it is very rare, some cases of the bubonic plague still occur today. Thankfully, doctors can treat it with antibiotics.

Estimated 25–50 million people died

The “Black Death” is also known as the bubonic plague—the same illness that caused the Justinian plague more than 500 years earlier. This time, the bubonic plague broke out across Europe, North Africa, and the Middle East.

As before, no one knew what caused it, how it spread, or how to cure it. Some believed the plague was a punishment from God. Some believed it was caused by “miasma,” or clouds of bad air.

When the plague returned in the 1660s, plague doctors wore bird-like masks
filled with herbs, vinegar, or dried flowers to protect against miasma. Treatments ranged from covering yourself in bread to taking cold baths. Unfortunately, none of these worked.

A very expensive treatment for the plague was a unicorn potion! This potion was made of ground-up unicorn horn mixed with water. Today, historians think these unicorn horns were actually narwhal tusks.

Estimated 48 million people died

When Christopher Columbus and other Europeans first arrived in North and South America, they met many different groups of Indigenous people already living there. As they began to interact, these two sides introduced each other to many new things—unfamiliar plants, animals, people, ideas … and diseases.

European ships brought many diseases from Europe, Asia, and Africa to the Americas—diseases that Indigenous peoples there had never faced before, such as measles, smallpox, typhus, and cholera. Because no one in the Americas had ever had these diseases before, no one had developed immunity, so many people got very, very sick. As much as 90 percent of the Native American population died.

Total death toll unknown, but an estimated 23 million people died in 19th-century India alone

The world has experienced no fewer than SEVEN cholera pandemics over the course of human history. Originally, many believed that cholera came from “bad air,” and only people with “low moral character” caught it. But a scientist named John Snow did not agree. He thought people caught cholera from drinking contaminated (unsafe) water. In 1854, he mapped where people were getting cholera in London and traced the outbreak back to one contaminated water pump.

He turned off the pump, and he was right! Disease rates dropped dramatically. Today, we know that a type of bacteria causes cholera, and people can catch it from contaminated food or water. Millions still get cholera every year, mostly people who don’t have access to clean water where they live.

Cholera was also known as the “Blue Death” because people’s skin would start to turn a bluish-gray color
as they grew sicker.

Estimated 20–50 million people died

The Spanish Flu had nothing to do with Spain. So why is it called Spanish Flu? This confusing name goes all the way back to the earliest days of the outbreak. Early newspaper reports in the U.S. claimed the sickness started in Spain, but they were wrong. Spain was just the first country to report it publicly. What we now call the “Spanish Flu” likely started in the U.S., probably at a military base in Kansas!

You may have had the flu before, but not this version. The 1918 flu outbreak was far deadlier and more dangerous than most, particularly for young adults.

Walt Disney got very sick from
the Spanish Flu when he was a teenager.

In the 1910s, many of the things doctors use today to prevent and treat the flu (such as flu shots and antivirals) did not yet exist. Still, people back then stopped the spread of the flu by many of the same tactics we still employ in modern times, like shutting down large gatherings and keeping sick people away from others until they recover.

About 200,000 people were paralyzed or died every year during the epidemic

During the polio epidemic, summer was a scary time to be a kid in the U.S. Polio can infect anyone, but it is particularly risky for children. The polio virus spreads through the sneeze or feces (that means poop!) of a sick person.

Just like now, it was incredibly important for people to wash their hands after using the bathroom or before eating.

Some people who got polio lost the ability to move their arms or legs. Some even stopped being able to breathe on their own. Because outbreaks of polio rose every summer and spread quickly from person to person, many kids felt afraid to do things they loved, like going to the public pool or playing with new friends. Many famous figures survived polio—including U.S. President Franklin D. Roosevelt, artist Frida Kahlo, and scientist Florence B. Seibert.

Eleanor Abbott invented the popular board game Candy Land while she was recovering from polio in the hospital. The game was so popular with the children in the hospital that she decided to pitch the idea to a toy manufacturer. And the rest is history!

Everything changed in 1955 when the first polio vaccine (invented by Dr. Jonas Salk) was released in the U.S. Within a year, cases of polio dropped from nearly 60,000 to less than 6,000. Improved sanitation also helped bring those numbers down.

By 1994, polio was officially declared eliminated from the United States.

Every single person in history has lived through dangerous outbreaks. And that, of course, includes you! Today, people across the globe face threats from many different infectious diseases, from illnesses that go back hundreds of years, like cholera and measles, to newer problems, like COVID-19.

These outbreaks can cause devastating harm, but there is good news! Thanks to the work of generations of people who faced, fought, and studied past outbreaks, we now know a lot more about infectious diseases than we used to. And that means we have better tools to prevent, detect, and protect people against the diseases around now and any that may emerge in the future.

Who knows? If you love science, maybe you’ll discover a cure someday!