How Contagious Is Ebola?

Epidemiologists study the spread of infectious diseases like Ebola. To determine how contagious a disease is, they use a measurement called the basic reproduction number, or R₀ (pronounced “R naught”).

“R₀ tells us how many people are likely to get a disease from someone who’s already sick,” says Abdul El-Sayed, an epidemiologist in Detroit, Michigan. “The bigger the R₀, the more likely the disease will infect a lot of people.” For example, a person with an illness with an R₀ of 4 will infect four people if nothing is done to prevent the spread of the disease.

Ebola has a relatively small R₀ of 2. This doesn’t mean it’s not a serious illness. But it suggests that Ebola won’t spread as fast as an infection with a high R₀, like chicken pox, which has an R₀ of 8.

Epidemiologists are scientists who study the spread of infectious diseases like Ebola. They use a special measurement to determine how contagious a disease is. It’s called the basic reproduction number, or R₀ (pronounced “R naught”).

“R₀ tells us how many people are likely to get a disease from someone who’s already sick,” says Abdul El-Sayed. He’s an epidemiologist in Detroit, Michigan. “The bigger the R₀, the more likely the disease will infect a lot of people,” he explains. For example, say a person has an illness with an R₀ of 4. That person will infect four other people if nothing is done to prevent the spread of the disease.

Ebola has a relatively small R₀ of 2. This doesn’t mean it’s not a serious illness. But it suggests that Ebola won’t spread as fast as an infection with a high R₀  would. Chicken pox, for example, has an R₀ of 8.

Although R₀ is a helpful tool, it’s not an exact measurement. “In reality, there are many situations that can affect the spread of an illness,” says El-Sayed. For example, vaccination programs in the U.S. have greatly reduced cases of measles and mumps, despite high R₀ values. Limiting patients’ direct contact with others can also curb a contagion. That’s why medical workers wear protective suits when treating patients with Ebola.

The last major Ebola outbreak spread across West Africa from 2014 to 2016. More than 17,000 cases and 11,000 deaths were reported. It even spread farther: 36 people were treated for Ebola in countries outside West Africa. That included cases in the U.S., Germany, Spain, and the United Kingdom.

Doctors are now working to contain the current outbreak to prevent a repeat of the last one—and save lives. “It is time for the world to take notice,” Tedros Adhanom Ghebreyesus, the chief of the World Health Organization, said at a news conference when declaring the current emergency.

Although R₀ is a helpful tool, it’s not an exact measurement. “In reality, there are many situations that can affect the spread of an illness,” says El-Sayed. For example, measles and mumps have high R₀ values. But vaccination programs in the U.S. have greatly reduced cases of those diseases. Limiting patients’ direct contact with other people can also help stop diseases from spreading. That’s why medical workers wear protective suits when they treat patients with Ebola.

The last major Ebola outbreak spread across West Africa from 2014 to 2016. Officials reported more than 17,000 cases and 11,000 deaths. The disease even spread farther: 36 people were treated for Ebola in countries outside West Africa. That included cases in the U.S., Germany, Spain, and the United Kingdom.

Doctors are now working to contain the current outbreak. They want to prevent a repeat of the last one—and save lives. Tedros Adhanom Ghebreyesus is the chief of the World Health Organization. He gave a news conference to declare the current emergency. “It is time for the world to take notice,” he said.