But I learned then that Ebola isn’t the fastest-spreading disease in human history. That distinction goes to measles. In the era prior to 1963, when children were first routinely vaccinated, each case of measles created 17 new secondary cases, with transmission spreading like wildfire in schools, especially. It was lethal in one in every three to four cases. At this rate, becoming infected with measles during childhood was inevitable and so were deaths.
Of course, this reproductive rate isn’t the whole story of epidemics. Each case of the Spanish flu – which caused the pandemic of 1918 to 1920 that many call the worst in recent history – produced two to five additional victims. Although that’s much lower than measles, Spanish flu was still able to spread worldwide because of the speed at which it moved, also known as its generation interval. Only two to three days elapsed between the first case and a generation of secondary cases. In other words, it doesn’t take long for the flu virus to settle into a new host and a substantial number of transmissions can occur even before a person realizes that he or she has the flu. An estimated 30 to 50 million people worldwide died from the Spanish flu.
The good news is that Ebola has a lower reproductive rate than measles in the pre-vaccination days or the Spanish flu. Our 2004 work, which produced the first estimates for Ebola’s reproductive rate by using mathematical modeling and epidemiological data from the Central African outbreaks, found that each case of Ebola produced 1.3 to 1.8 secondary cases on average. This ongoing outbreak, a colleague and I recently found, has a reproductive rate that is about the same as the last one. It hasn’t become more transmissible in the more than 10 years it was lying low — and humankind has experience in dealing with it.
And the time that elapses between the first Ebola case and the generation of secondary cases is about two weeks. This should allow plenty of time to identify those who are sick and protect people who might come in contact with them. People with Ebola are contagious and able to transmit the virus only when they are showing symptoms, which occurs about a week after they are exposed to the virus.
To break the chain of the current Ebola epidemic, our numbers show that health-care workers need to stop about 50 percent of infectious contacts by effectively isolating people who are infectious. (Vaccinating at least some of the population would be another option, but no licensed vaccine is available.)
The trouble is that the countries suffering from outbreaks have weak health-care systems – perhaps too weak to halve the number of infectious contacts.
These countries lack gloves, gowns, face masks and other essential supplies to protect nurses and doctors from infection, and they don’t have an adequate surveillance system to catch and identify Ebola cases in a timely way. The number of doctors and health centers is small as well.
As a result, Ebola has spread with very little resistance and the rate of new infections from month to month has risen in Liberia and Sierra Leone, in particular. The rate for new cases of Ebola in Guinea has not risen as much, which suggests that Liberia and Sierra Leone have missed badly needed opportunities to control the outbreak.
Nigeria’s ability to break the chain of Ebola also is worth noting. Health authorities detected the first Ebola case in their countries only three days after that person’s arrival in Nigeria from Liberia by airplane, and acted quickly to isolate infectious individuals and trace their potential contacts. Nigeria has observed 20 cases in which Ebola was transmitted, with no new cases reported as of Sept. 8.
Until this year, textbooks have described Ebola as a type of infectious disease that could cause no more than a few hundred cases in hard-to-reach forested areas of Central Africa. Little attention or funding has been focused on Ebola, unfortunately, because health policymakers thought it would never escape its remote home base and infect large numbers of people. But while the virus has not changed much in more than a decade, many parts of Africa are very different. People live in much denser quarters where a new disease is now more likely to take off. The population also is much more mobile than ever before. This increases opportunities for the virus to reach areas where new chains of transmission can grow and become established.
Math and history show us that decisive efforts to isolate those who are infected with Ebola and to follow up quickly with the potential contacts of the infected can help control an epidemic. We’re lucky that we have such capacities in the United States; if Ebola were introduced here, it could not get much of a foothold. But our world is interconnected in ways it never was before, and diseases that require substantial contact to spread aren’t the only things circulating. If a virus such as Ebola can quickly flare out of control, consider the impacts of a novel strain of influenza.