When I read Richard Preston’s The Hot Zone in the mid-1990s, I was terrified. This was the first I had heard of a scary new disease called ebola. I was working for an AIDS Service Organization at the time, so I understood — better than most — how blood-borne infectious diseases are contracted. Still, the images of how the victims of this virus die are still with me. Horrifying.
But I’m not at all afraid of ebola today. Not one little bit. Why? Math.
It’s difficult for ebola to spread. Really difficult. Like HIV, the ebola virus only lives in bodily fluids, including blood, saliva, mucus, vomit, semen, breast milk, sweat, tears, feces and urine. (HIV is only transmitted through four bodily fluids: semen, vaginal fluids, breast milk and blood.) Transmission can occur when infected bodily fluids come into contact with a person’s eyes, mouth or nose, or an open wound or abrasion.
Compare this to measles, which is transmitted through the air. The measles virus lives in the mucus lining of the nose. A sneeze or cough can release virus-infected droplets into the air. Breathe in the air with little measles droplets, and unless you’ve been vaccinated, it’s very likely you’ll see a tell-tale rash in a few days.
Since measles is highly contagious for four days before symptoms appear, a person can transmit the virus without even knowing he has it himself. According to the CDC, measles is so contagious that if one person has it, it will spread to 90 percent of the people who come in contact with that person (if they are not already immune, thanks to the vaccine).
It’s All About the R0
The way a virus is transmitted helps determine how contagious the disease is. And the big deal here is something called R0 or “reproduction number” (also called “r-naught”). R0 is the number of people that one infected person will likely infect during an outbreak.
Those of us of a certain age might remember a shampoo commercial that illustrates this perfectly.
Like Fabrerge Organics shampoo, ebola’s R0 is 2. When one person contracts ebola, it is likely that two others will become infected. Yes, those numbers add up — and they have in parts of Africa.
Now take a look at measles, with an R0 of 18. When one person gets measles, it’s likely that 18 people around him do too. Then each of those 18 people spread the virus to 18 more people. In one generation of this infection, 18 x 18 (324) have contracted measles. That’s compared to only 2 x 2 (4) people who will likely contract ebola in one generation of the infection. In fact, measles is still one of the leading causes of death in children around the world. According to the WHO:
Measles is still common in many developing countries – particularly in parts of Africa and Asia. More than 20 million people are affected by measles each year. The overwhelming majority (more than 95%) of measles deaths occur in countries with low per capita incomes and weak health infrastructures.
But measles is not a major threat in the U.S., and we all know why — the measles vaccine. Ebola has no vaccine, but a relatively strong health care system in our country and its very low R0 makes ebola a low threat, compared to other viruses, like HIV and certain strains of influenza.
The scary thing about ebola is not how quickly it spreads but how basic medical care can keep it from spreading. We have that basic care here in the U.S. Large swaths of Africa do not.
And along with a low R0, the ebola virus has a relatively short infectious period — about a week. On the other hand, HIV is infectious for years and years — many of those years while the infected person has no symptoms or does not even test positive on an HIV test. The relationship between time and infection matters, too.
You Should Worry About Other Things Instead
For example, the National Institutes of Health (NIH) reports that each year, about 5,000 people under the age of 21 die in alcohol-related incidents, including car crashes, falls, burns, homicides, suicides and alcohol poisoning.
According to the Federal Reserve, Americans held $229.4 billion in consumer credit (outstanding household debt, including credit cards and loans) in July 2014.
The global sea level is rising at alarming rates, according to the National Oceanic and Atmospheric Administration (NOAA). Before 1900, these levels remained constant. Since 1900, the levels have risen 0.04 to 0.1 inches per year. But beginning in 1992, that rate climbed to 0.12 inches per year. This translates to much greater likelihood of flooding in coastal areas (including the neighborhood where I lived for 10 years).
And we should be concerned about ebola in Africa, mainly because we can do something about the higher rates of ebola infection and deaths there.
But ebola in the United States? Really, this shouldn’t be a worry for you. Let the math ease your mind.
Photo Credit: CDC Global Health via Compfight cc