The ongoing outbreak of Ebola is a potentially global catastrophe, currently affecting multiple countries in West Africa. Concerns over the epidemic have been exacerbated by the emergence of travel-associated cases of the disease — a patient was diagnosed in the United States, and subsequently a healthcare worker who provided care to this patient also tested positive for Ebola. The United States Centers for Disease Control (CDC), World Health Organization (WHO), and other national and international health agencies are all on high alert, and news organizations encourage the general public to exist in a state of fear over the potential of a global Ebola pandemic.
But just how much of this concern is justified?
To answer that question, we need to consider the nature of pandemics generally, from a historical perspective, as well as the specific properties of the Ebola virus in this historical context.
So how do pandemics usually happen? Popular media and movies such as Contagion would have you believe that some mysterious virus emerges from the heart of the rainforest’s darkness to blight humanity and threaten global populations. But is there any historical basis for this kind of pandemic, or is it simply a scary movie plot?
If we look at historical pandemics, the answer is that pandemic diseases usually have far more pedestrian origins, and are rarely if ever emerging pathogens. The Athenian plague, the Black Death, the 1918 pandemic flu — none of these were caused by emerging pathogens. Instead, they were caused by pathogens that have circulated in humans for some time, and that are well adapted to spreading in human populations.
For a pathogen to cause a pandemic successfully, it needs to be able to turn its human host into an efficient system for propagating the pathogen. Whether through sneezing, coughing, or passing watery stool, a pandemic virus or bacteria needs to be able to spread efficiently from one host to many.
This requirement of being able to spread efficiently from one host to many is at odds with the idea of a pandemic emergent disease — generally speaking, emerging pathogens that have not before spread in humans are not very good at human-human transmission, as they have not had the time to evolve in humans that would be required for such spread to be efficient.
Indeed, scientists are unaware of any virus that has dramatically changed its mode of transmission, despite fears of such a shift happening with such viruses as avian influenza and Ebola. Avian influenza has circulated in birds for over 20 years and has never developed human-human transmission, and despite concerns, there is no evidence that ebola will ever develop airborne transmission — this is pure speculation, and not borne out by the historical record of pandemic disease.
Simply put, Ebola as an emerging pathogen is not a good candidate for causing pandemic disease, especially in the developed world, and it is unlikely to do so based on its route of spread.
Ebola spreads by direct contact with an infected person’s bodily fluids, and does not spread by the airborne route. As such, its reproductive index, or the number of individuals who can be newly infected per individual who contracts the disease, is quite low. For every infected person, only one or two new individuals will contract the disease.
This very low reproductive index means that surveillance, containment, and quarantine are very effective at controlling Ebola and preventing its spread. In developed countries such as the United States and most of Europe, concern over a pandemic seems misplaced.
However, there are countries not adequately prepared to contain even a virus with such a low reproductive index, and these countries are at risk. It is in these places, places with refugee camps, densely populated slums, and poor sanitation, that Ebola could truly wreak havoc. The WHO and CDC are dispatching personnel to these places, focusing their efforts on surveillance so that they might be able to contain Ebola’s spread.
So, how do we stop pandemics from occurring? The best answer is to avoid putting too many people so close together that even a pathogen with a very low reproductive index can become dangerous. Refugee camps, slums, and dense, poor urban centers are all potential disease factories, allowing viruses to jump quickly from person to person. A state of fear solves nothing — but social change might.
As for the current Ebola scare, a pandemic in the developed world is unlikely, but we must be vigilant that poorer countries are not affected too harshly by the outbreak.
