Metro

Evan Horowitz

What are your chances of getting Ebola?

While fears of Ebola have waned here in the US, the outbreak continues to spread through West Africa. To date, about 6,000 people have died. And while the disease has been brought under control in many areas, the World Health Organization acknowledges that there are places where it is still outpacing the international response.

If you are wondering about the risk of a global pandemic with scenes reminiscent of movies like “Outbreak” and “28 days later,” here are two things to keep in mind.

RELATED: Few preparations in event of Ebola in US

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First, while Ebola is an extremely virulent disease, its impact pales in comparison to other global killers like measles, AIDS, or even the flu.

Second, the likelihood of Ebola spreading across the US is vanishingly small. It isn’t a particularly contagious disease, and in a developed country with strong health infrastructure, it probably wouldn’t spread much at all.

What is Ebola?

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Ebola is a virus, like the flu or the common cold. In fact, the official name is Ebola Virus Disease. It was first identified in the 1970s, and since that time, there have been dozens of Ebola outbreaks, most of them in Central and East Africa.

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The early symptoms of Ebola are flu-like, including fever, weakness and muscle pain. Quickly, though, this progresses to vomiting, diarrhea, organ failure, and spontaneous bleeding (internal and external).

Do most people die?

In the current outbreak, well over half of all infected people have died. Previous outbreaks have seen death tolls as high as 90 percent and as low as 25 percent.

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Ebola comes in different strains (think of swine flu and bird flu), and these have different mortality rates. The type currently spreading through Liberia, Guinea, Sierra Leone, and now Nigeria is the Zaire strain, which is not just the deadliest form of Ebola but one of the deadliest diseases on earth.

How do you get it?

Ebola isn’t that contagious. Unlike the flu, for instance, it doesn’t travel through the air. You can only get it from direct contact with the blood or other bodily fluids of infected people.

One reason it has been spreading through West Africa is that in some communities, it is common for mourners to touch the bodies of deceased loved ones before burial, and people who’ve succumbed to Ebola are still contagious after death. Health workers, too, face increased risk, because caring for Ebola patients requires close contact. This is likely how the two infected US doctors contracted the disease.

Generally speaking, though, Ebola doesn’t spread easily. People who study disease transmission tend to look at two major variables when predicting how far diseases will travel.

Generation time. This is a measure of how long it takes for one person to infect another. If you’re sick and you pass your germs on to someone else, will they get sick the next day? A week later? A month? With flu, the generation time is very short, just 2-4 days, which is one reason flu can spread so quickly through a group of people. Ebola has a much longer generation time, upwards of 2 weeks. That slows down transmission.

RELATED: Vaccines not available for Ebola, but some being developed

Reproductive number. If you’re sick and you nonetheless decide to go about your everyday routine how many people are you likely to infect? That’s what the “reproductive number” measures. When you walk around with the flu, you’re likely to infect two people. If you have measles, it’s seven or more. With Ebola, it’s roughly 1.5 to 2.

Putting these together, you see that Ebola scores relatively low on both these measures of infectiousness, meaning there isn’t a clear mechanism for rapid spread. Flu, for instance, may have a low reproductive number but its speedy generation time still gives it a means to move quickly through a population. And while Smallpox had a generation time quite similar to Ebola, it jumped much more readily from person to person.

Reproductive numberGeneration time (days)
Ebola1.5-214-21
Flu1.5-22-4
Dengue1.3-6.319-24
SARS1.2-3.68.4
Rubella2.9-7.815-23
Smallpox3.2-6.914-20
Cholera4-157-10
Measles7.1-29.39-17

Source: Infectious Disease Epidemiology: Theory and Practice. Also, Dr. Donald Burke and Dr. Justin Lessler.

Are there any treatments?

At this point, there’s very little doctors can do for Ebola victims, beyond keeping them hydrated. There are some experimental treatments at different stages of review, including a vaccine and some medications, but nothing proven or approved.

One reason there are currently no treatments is that, horrific as the disease may be, it mainly affects poor people in poor communities. That makes it harder for drug companies to earn money from potential products, and thus reduces their incentive to spend money on research. Of course, money isn’t the only incentive, and there are other ways to encourage drug research, including medical prizes, but for now finding a working treatment remains an open challenge.

On a scale of one to 10, how worried should I be?

If you live in the US your worry-level should be quite low. Ebola doesn’t have the disease profile to spread in a place with reasonable medical resources.

If you find yourself in West Africa, you have more cause for concern. Developing nations lack the public health resources needed to properly quarantine people, provide health workers with protective gear, and educate the population about risks and best practices. The deadliness of this Ebola outbreak has as much to do with the social and political structures in West Africa as with the structure of the virus itself.

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And yet even if you do live in a country where poor public health infrastructure makes you more vulnerable to Ebola, there are other diseases you should be much more worried about.

UNICEF estimates that 9.7 million children under 5 die each year of preventable diseases. That’s 25,000 every day, compared to the 6,000 who have died from this Ebola outbreak--the worst outbreak ever. Across the globe, a bad flu season can kill over 250,000 people. Measles and cholera both kill over 100,000. Diarrheal diseases kill 1.5 million. In terms of global killers, Ebola just isn’t in the same class.

Still, while there are other, more infectious illnesses, Ebola remains a deadly disease that’s killing thousands of people and wreaking havoc across West Africa. That’s the broader context for the current crisis. It may not be the beginning of a global pandemic, but without better treatment and more support for public health services this flare-up could continue, and future outbreaks will remain largely unavoidable.

Update: Total number of deaths in this Ebola outbreak updated on Dec. 3.

Evan Horowitz digs through data to find information that illuminates the policy issues facing Massachusetts and the U.S. He can be reached at evan.horowitz@globe.com. Follow him on Twitter @GlobeHorowitz
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