The Right Way To Think About Your Risk Of Ebola

Ebola has riveted our attention: It’s a deadly disease with no known cure, and as is true of most infectious diseases, it’s easy to imagine how it could become a global pandemic and threaten us directly. For what it’s worth, though, here is what the federal Centers for Disease Control and Prevention tells us about Ebola:

  1. The Ebola virus is not spread through:
    • Casual contact
    • Air
    • Water
    • Food grown or legally purchased in the U.S.

2. How do you get the Ebola virus? Direct contact with:

    • Body fluids of a person who is sick with or has died from Ebola (blood, vomit, urine, feces, sweat, semen, spit, other fluids)
    • Objects contaminated with the virus (needles, medical equipment)
    • Infected animals (by contact with blood or fluids or infected meat)

3. Early symptoms include:

    • Fever
    • Headache
    • Diarrhea
    • Vomiting
    • Stomach pain
    • Unexplained bleeding or bruising
    • Muscle pain

4. When is someone able to spread the disease to others?

    • Ebola only spreads when people are sick. A patient must have symptoms to spread the disease to others.
    • After approximately 21 days, if an exposed person does not develop symptoms, they will not become sick with Ebola.

These are the facts, and they’re well-established. Unfortunately, another well-established fact is that what people believe—and therefore fear—is influenced by many things other than facts.

For example, we all tend to arrive at our beliefs about the frequency with which things occur not from statistical analysis but from the ease with which we can recall examples of their happening. So if we’ve recently heard news about an airplane crash, we’re likely to believe that the likelihood of the airplane in which we’re about to fly crashing to be greater than statistics suggest.

And if we hear about a few nurses becoming infected with Ebola, we’ll believe the likelihood that we’ll contract it is higher than it actually is.

We all tend to believe stories more than facts. And when faced with the need to make a decision—to start a medication, to have surgery, to get a flu shot—far more often than not (and mostly without consciously realizing it) we rely on our emotional beliefs about the risks and benefits. And our emotional beliefs draw mostly on our experience and the stories we tell ourselves about it.

“My wife’s sister’s boyfriend took that pill and had a terrible reaction. There’s no way I’m going to take it!” one patient tells us. “Dr. X operated on a friend of mine and he’s been in pain ever since. No way I’m letting that guy touch me!” says another. “I’ve seen this drug advertised on television. What do you think about me trying it instead of what you suggest?” asks a third.

We believe thinking this way leads us to make wise decisions—to fear the right things—but it doesn’t. We hear about a friend or relative having a known complication as a result of a surgery and decide as a result that we don’t want the surgery—even though the statistical likelihood of such a complication happening to us is less than one percent; even though that complication has never happened to any of our own surgeon’s patients; and even though more than one doctor has told us that we need the surgery ourselves.

Or we read about the side effects of a drug our doctor recommends and decide we don’t want to take it even though studies show that the risk of those complications is far lower than the likelihood that it will treat our symptoms or even prolong our life.

Or we read about thousands becoming sick in Africa with Ebola, plus a few cases in the U.S., and imagine the threat to ourselves is more immediate than it actually is.

We’re not suggesting that we shouldn’t be concerned about Ebola in the U.S. or other countries, or that circumstances couldn’t rapidly change to make the risk of contracting the disease here rise dramatically. We’re saying that when deciding what we should most fear, when deciding just how concerned we should be, we need to critique our own thought process mercilessly and avoid anecdotal thinking.

To think statistically is to calculate the true likelihood that something bad (or good) will happen to us. And in far more cases than most would believe, we do have information that allows us to do so.

We know, for example, that the likelihood of any one of us becoming the victim of a terrorist attack is about 1 in 20 million. But think: Is your fear of terrorism proportional to that statistic or to the frequency with which you hear about terrorism on the news? Another example: You should be far more afraid of driving a car than flying in an airplane. Not only are car accidents statistically more likely than airplane crashes, but most of us drive far more often than we fly, thereby exposing ourselves to the risk of a car accident far more often than we expose ourselves to the risk of an airplane crash. But how often do you worry about getting in an accident when you take your car? We’re not suggesting you need to; in fact, we’re arguing the opposite: that because of our exposure to anecdotes, we often worry far more than we should about things whose statistical likelihood is actually small—and, conversely, not enough about things whose statistical likelihood is actually large.

The likelihood that any one of us will contract Ebola in the U.S. right now can be calculated broadly by dividing the number of patients who contracted Ebola in the U.S. without traveling to Africa (2) by the total population (317 million). When you do this, you get a number too small to describe in any meaningful way. Of course, the risk may increase as travel between countries continues. But keep in mind:

  1. The greatest risk is to healthcare workers who care for patients with symptomatic cases.
  2. Even if, say, 100,000 people in America came down with the disease, the risk of anyone else contracting it would remain 100,000 divided by 317 million, or just 0.03%.

To put this in perspective, consider that your risk of contracting influenza lies somewhere between 5 and 20%. Yes, Ebola has a much, much higher mortality rate than influenza. But your likelihood of getting Ebola is also much, much lower than your risk of getting influenza.

Of course we want the CDC to take all the right steps to prevent our risk from rising—and many of us will feel, rightly or wrongly, that we have good cause to criticize the job our government is doing, now and in the weeks and months ahead. But how many of us will look at the things we should really fear, like influenza, and do what we can ourselves to reduce the risk of those things—like, say, getting a flu shot?

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  • Thanks for adding a voice of sanity to the Ebola scare. People are so afraid of this brutal disease it’s hard to keep calm about it. Even though I’m nowhere near anyone who’s been infected, I worry. But I’m able to laugh at myself and remind myself that it’s because I read about it every day. If I start getting psychosomatic symptoms, I’m going to stop reading the news and read a funny book to cure myself.

  • Thank you for your wise words.

  • Thank you, Doctor, you provide a much needed service. This is a drum that needs beating.

  • Rational decision-making seems to be in very short supply these days, with a lot of things. Even the nurses and doctors in Dallas’ Texas Health Presbyterian hospital ignored some very clear warning signs when discharging Mr. Duncan when he admitted himself the first time——weeks after Ebola had been daily front page news.

    But even simple information can seem confusing. Touching a sweaty person on the arm sounds like pretty casual contact to me. But casual contact is in one category in your article, and coming in contact with the sweat of a sick person is in another.