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How Contagious Is Ebola?

The deadly disease is back, but doctors are prepared to fight it

Sven Torfinn/Panos Pictures Panos/Redux

An Ebola response team disinfects the house of a resident who came in contact with an Ebola victim.

First your fever spikes, your joints hurt, and your head aches. It feels like the worst flu you’ve ever had. Then it gets worse: You double over with stomach pains, vomiting, and diarrhea. For about 60 percent of victims, these symptoms lead to organ failure and, ultimately, death.

This virus is called Ebola, and there’s been an outbreak of it in the Democratic Republic of the Congo since August 2018. As of press time, more than 2,700 people had become sick and 1,800 had died in this central African country. In July, the World Health Organization declared a public health emergency after the first cases were reported in Goma. This city has a population of about 1 million, which could help spread the disease faster.

Jim McMahon/Mapman

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 Rof 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.

Callista Images/Alamy Stock Photo

The Ebola virus is spread only through contact with blood or other fluids of a sick person.

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.

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Watch a math helper video about exponents.

Use exponents to answer questions about the spread of different diseases, assuming no preventive measures are taken. Record your work and answers on our answer sheet.

Ebola has an R0 of 2. How many third-wave cases can doctors expect?

Influenza has an R0 of 3. How many fourth-wave cases can doctors expect?

Chicken pox has an Rof 8. How many fifth-wave cases can doctors expect?

Smallpox has been eradicated in nature, but it caused the deaths of at least 300 million people—the most of any disease in history. Smallpox had an R0 of 7. How many sixth-wave cases would doctors have seen before it was eradicated?

How many total cases of Ebola could doctors expect after a fifth wave of infection? (Hint: Add the number of cases in each wave.)

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