Chickenpox is a highly contagious disease caused by the varicella-zoster virus that leads to itchy skin eruptions, which are sometimes compared to a “dew drop on a rose petal.”

Until the development of a chickenpox vaccine in the late 20th century, the disease was a common childhood illness that could cause serious health problems in people who didn’t contract the disease until adulthood. More than four million people got chickenpox every year in the United States, resulting in more than 10,000 hospitalizations and 100 deaths. Since vaccinations began, those numbers dropped significantly.

The CDC reports that fewer than 350,000 people contract the disease per year, and that there are fewer than 1,700 hospitalizations and 20 deaths annually from chickenpox.

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There’s evidence of chickenpox dating back to ancient times, and the earliest known use of the term “chickenpox” dates to 1691—although it’s not clear how it got this name. It's believed the disease was brought to the Americas in the 15th century by European explorers and settlers. Once on the continent, it (and other diseases) spread among Native Americans since Indigenous people had not previously been exposed to the virus.

Before the 18th century, diseases that appeared to produce “pox,” or skin eruptions, were commonly lumped together. This included chickenpox, smallpox and syphilis, which was known as “large pox” or the “great pox.” The first scientist to provide a detailed description of chickenpox differentiating it from smallpox was the English physician William Heberden. In 1767, he noted the physical differences between the two diseases, and also recorded that people who’d had chickenpox “were not capable of having it again.”

It wasn’t until later that scientists realized chickenpox was related to shingles. In the late 19th and early 20th centuries, Hungarian pediatrician James von Bokay observed several instances in which younger people seemed to contract chickenpox after being exposed to someone with shingles, a disease that can cause nerve damage if not treated properly. This led him to suggest that there was a link between the two diseases.

Scientists later confirmed this theory by discovering that after a person recovers from chickenpox, the varicella-zoster virus stays in his or her body, and can cause the person to develop shingles later on.

In the 1950s, scientists isolated the varicella-zoster virus for the first time, paving the way for efforts to vaccinate against chickenpox and shingles. After that, it took several decades to develop and distribute vaccines for these illnesses. The U.S. Food and Drug Administration approved the first chickenpox vaccine in 1995 and the first shingles vaccine in 2006.

Compared to other childhood vaccines, the chickenpox vaccine was a relatively late development. Maurice Hilleman, who helped develop a measles vaccine in the 1960s, had also tried to push for a chickenpox vaccine around that time. However, diseases ended up receiving higher priority depending on the rate of death and disability associated with them, writes epidemiologist René Najera, editor of The History of Vaccines, an online resource by The College of Physicians of Philadelphia, in an email to HISTORY.

“As a result, chickenpox fell toward the bottom of the list because it is a relatively mild disease in children,” he says. As new vaccines helped control more severe childhood diseases, chickenpox moved higher up on the list.

The CDC estimates that a person with chickenpox can spread it to up to 90 percent of the people with whom they come into contact who haven’t previously had chickenpox or the vaccine. In addition, the period in which a person is contagious lasts for several days. It begins one or two days before the chickenpox eruptions begin to show, and lasts until all the fluid-filled skin lesions have scabbed over. Typically, chickenpox lasts for 4 to 7 days.

Before the vaccine, chickenpox spread easily in households and classrooms, and was especially dangerous for adults who had never had it. Both children and adults may experience fever, fatigue and body aches with chickenpox, but in adults these symptoms can be more severe. Adults are 25 percent more likely than children to die from chickenpox, according to the National Foundation for Infectious Diseases. The disease can lead to health complications like bacterial infections, swelling of the brain and pneumonia.

Although the chickenpox vaccine has greatly slowed the spread of the disease in schools, outbreaks occur in some parts of the United States where parents have declined to vaccinate their children. This is similar to the way that childhood diseases like measles, which went from common to uncommon in the late 20th century, began to break out in schools again in the 21st century.

Still, with the widespread adoption of the chickenpox vaccine, the disease “has joined polio and measles in the list of infectious diseases that are candidates for eradication,” Najera says. So far, the only human disease that vaccines have globally eradicated is smallpox, but scientists and doctors hope to one day add more to the list of diseases that have been vanquished by vaccines.