What is the Scariest Disease?

By Beth Skwarecki on at

Whether you're personally afraid of Ebola or not, you have to admit it's a scary disease: no vaccine, no cure, and high fatality rate are just a few of its distinguishing features. Recently I polled my friends on what diseases they were afraid of, and many of the ones that made the list were things like cancer and alzheimer's, conditions that can't be easily prevented or cured, and that have a high likelihood of developing agonising symptoms.

While public health priorities should focus on the largest or fastest-growing threats, fear is a personal thing that doesn't always match up with objective numbers. Influenza kills more people each year than Ebola ever has, but that doesn't automatically make it scarier. Here, I'll take a look at a few ways to rank how "scary" a disease might be:

  1. How likely am I to die of it?
  2. If I catch the disease, is death inevitable?
  3. How contagious/infectious is the disease?

How Likely am I to Die from It?

Total fatalities are a good place to start, if only to show off how backwards our intuition can be. If something is responsible for a lot of deaths in absolute numbers, it means it's a high percentage of total deaths, and thus, more likely that you will die of it. So what are the major killers worldwide?

Only three of the top ten are infectious diseases: lower respiratory infections (think pneumonia), HIV/AIDS, and diarrheal diseases, which include the likes of rotavirus, E. coli, and cholera. If you want to rank specific germs, the top three are:

  1. HIV (AIDS) – 1.6 million deaths in 2012
  2. Mycobacterium sp. (tuberculosis) – 1.46 million deaths in 2013
  3. Plasmodium sp. (malaria) – 627,000 deaths in 2012

Selected honorable mentions, for comparison:

  • Influenza: estimated range of 250,000-500,000 each year (there is a vaccine for this, about 60 per cent effective)
  • Pertussis: 195,000 deaths each year (there is a vaccine for this, 80-90 per cent effective depending on age but decreasing over time)
  • Measles: 122,000 deaths each year (there is a vaccine for this, 95 per cent effective)
  • Ebola can't keep up in this category. Through 2013 it caused only 41 deaths per year. The current outbreak stands at 4,877 deaths as of October 22, although if it's not contained it has the potential to climb high on our list. See this essay on the potential for Ebola to become endemic in Africa. You want scary? That's scary.

That said, your chance of dying of anything is only about 1 per cent per year. Let's keep our focus on infectious diseases and look at a scarier statistic…

If I Catch the Disease, is Death Inevitable?

There's a metric for that: the case fatality rate, or CFR. (As in, how many cases of this disease result in fatality?)

Several diseases have horror-movie-ready CFR's of 100 per cent or close to it:

  • Creutzfeld-Jakob (the human version of mad cow)
  • Kuru, another prion disease
  • Naegleria, the brain eating amoeba
  • Rabies (untreated—the typical treatment is to administer rabies vaccine after exposure but before the patient starts showing symptoms. There are no official numbers on the effectiveness of this vaccine, but anecdotally it's close to 100 per cent).

Some other high CFR diseases:

  • Inhalational anthrax – 93 per cent
  • HIV, if untreated, in developing countries – 80-90 per cent mortality within 5 years
  • Ebola – 71% in the current outbreaks, according to the best estimates.
  • MERS-CoV, an emerging disease that is related to SARS and associated with camels: 45 per cent

Does Ebola's CFR surprise you? The current outbreak has twice as many cases as deaths, which would seem to put the CFR around 50 per cent. But it's tricky to calculate CFR for outbreaks that are ongoing and even growing, since the total cases include people who will die, but haven't yet. Example: say you open an Ebola clinic and admit ten patients, seven of whom will die. If nobody is dead by the end of the day, you might say the CFR is 0 per cent. Next week, if five of them have died, you would calculate the CFR at 50 per cent, and in the meantime maybe you're admitting five new patients, which instantly drops the rate to 33 per cent (5 out of 15). But if you track those original ten patients over time, you'll get the right answer: 70 per cent. Here is a discussion of current estimates of Ebola's CFR.

How infectious/contagious is it?

Let's be clear: these are two different questions. Something is very infectious if it takes very few germs (virus particles, bacterial spores, etc) to trigger disease. Ebola is extremely infectious; so is inhaled anthrax.

A disease's contagiousness, on the other hand, doesn't count individual microorganisms, but rather describes how quickly it spreads. Say you share a funny cat picture on facebook and it's so good that ten friends post it on their walls. Ten of each of their friends post it, and ten of theirs, and so on; that picture will eventually be all over the internet. But if you share a funny picture of your lunch instead, and most people who see it (are "exposed" in epidemiology speak) don't bother to pass it on, that meme will soon fizzle out.

In epidemiological terms, our funny cat picture has a basic reproduction number, or R0 (pronounced "R naught") of 10. Our lunch picture, somewhere near zero. An infectious disease needs at least an R0 of 1 to spread; that would mean each person spreads it to one other person. Ebola's R0 is somewhere around 2; think of the Dallas patient that spread the disease to two health care workers. That's a typical case.

Measles clearly gets the gold in this contest, with an R0 of up to 18. That means that, in many outbreaks, each sick kid was infecting an average of 18 friends. Here's a ranking from Wikipedia, taken from published data on each entry:

Values of R0 of well-known infectious diseases

Disease Transmission R0
Measles Airborne 12–18
Pertussis Airborne droplet 12–17
Diphtheria Saliva 6–7
Smallpox Airborne droplet 5–7
Polio Fecal-oral route 5–7
Rubella Airborne droplet 5–7
Mumps Airborne droplet 4–7
HIV/AIDS Sexual contact 2–5
SARS Airborne droplet
Influenza(1918 pandemic strain) Airborne droplet
Ebola(2014 Ebola outbreak) Bodily fluids

Important public health message: note that the top seven diseases on this list are vaccine-preventable. If you get your kid the MMR and DTaP-HepB-polio shots, that's all seven (plus tetanus and hepatitis B as a bonus) prevented with just two jabs of the needle. Not a bad deal.

Here's a good explainer on the difference between infectious and contagious(and, bonus: the difference between isolation and quarantine).

Bottom line

Which diseases are the scariest? It depends on what scares you. Ebola ranks high for its case fatality rate alone. HIV made all of our short lists, and it's still not preventable, although treatments are available that can mitigate symptoms for years. Heart disease and stroke are more likely to kill you than any disease, but if you have a choice of what to catch, your odds are better with the flu than with kuru or brain-eating amoebas, which are, thankfully, rare. But ultimately, it's up to you to decide what disease should be the star of your next nightmare.

Fun fact: The disease in the 1995 movie Outbreak was not Ebola, but the fictional "Motaba virus." Symptoms were Ebola-like but it had a 100 per cent case-fatality rate and airborne transmission. I'm unaware of any calculations of its R0; anybody want to watch it and run the numbers?

This article first appeared on PLoS Blogs and republished under Creative Commons license. Ebola image by CDC.