Epidemiology: How to know when things are safe and when they R-naught

As the ebola epidemic continues to rage in west Africa and new cases start to pop up in countries like Germany and the US, people are beginning to pay attention to the study of how diseases spread through groups.  Movies like Contagion  and Outbreak give some idea of the work that gets done when a disease begins to pose a threat, but not many of us really understand the nuts and bolts of the science known as epidemiology. Where do diseases come from? How are they spread? Why has the current ebola epidemic proven to be so difficult to contain if, as we learned a few articles ago, the disease isn’t actually all that easy to catch? It turns out the answers are as interesting as the questions.

Where do diseases come from?

How can a disease like ebola suddenly burst onto the world’s stage? When you stop and think about it, it doesn’t make a lot of sense. If ebola is so deadly and is able to spread from person to person the way it has been for the past 10 months, why isn’t it something that is always going on? Well, in the case of ebola and many other diseases that harm people, the reason is that diseases have reservoirs where they hide out between epidemics.

When I say "reservoir" I don’t mean that there is a dam somewhere and behind it is a churning green soup of ebolavirus. Disease reservoirs are animals that viruses can live within without causing any ill effects. In the case of ebola, the reservoir is thought to be fruit bats. The reservoir for influenza is sea birds. The reservoir for plague is fleas. Every animal out there is a potential host for the next horrifying pandemic. Every so often humans come into contact with these animals in a way that allows diseases to jump into our bodies, this is called a spillover. For more on that, check out this great book published last year by David Quammen. Diseases that spread this way, from animals to people are called zoonoses (plural of zoonosis) and they make up most diseases you can name.

How do diseases spread?

The spread of a disease through a group of people depends on a number of things. Epidemiologists bring a few different factors together to present that information in a neat and tidy number they call R­­₀ (pronounced “R-naught” because being British is fun). In its simplest terms R₀ is the average number of healthy people that a sick person will infect while they have the disease. Ebola has the same R₀ as hepatitis C: 2. That means that if I had ebola I could expect to infect 2 new people before I either died or was cured, maybe my wife and my doctor, or a doctor and a nurse. Either way, a couple of you suckers are going down with me. R₀’s of other notable viruses include 4 for HIV and SARS, 10 for mumps, and a whopping 18 for measles.

R_{0 ­} depends on a few different things: the probability of infection after being exposed to an infected person, the average rate of contact between infected and susceptible people (some people will be naturally immune), and how long the disease is contagious for. What is working against ebola’s R_{0 ­}are the facts that the rate of transmissible contact between people is low (you need to be in physical contact with a person’s bodily fluids to catch it), and the fact that the disease is only contagious when a person starts to show symptoms (which usually isn’t very long… because ebola kills too quickly). Diseases with higher R_{0 ­}are able to spread through the air or survive in water.

Why haven't we been able to stop ebola yet?

Aside from the actual treating of sick people and coordinating quarantines and such, epidemiologists are also disease detectives. It is their job to determine who the first person to catch the disease was in an outbreak (the infamous "patient zero") and by what means it was able to spread. The full story of the current ebola outbreak can be found in Jeffery E. Stern’s article Hell in the Hot Zone, published earlier this month by Vanity Fair – it is definitely worth reading.

The Cliff Notes version is that cutting down the rainforest in West Africa brought people into closer contact with bats, leading to the spillover. After that, the international response was swift and well-coordinated, but it was not communicated well enough. The problem seems to have been that the teams of doctors and scientists in hazmat suits that rolled into the afflicted villages did not tell the friends and family of patients what was going on in a way that they could understand and trust. All those people saw was their loved ones being carried into tents by people in space suits and disappearing forever. After that people got understandably scared of western doctors. They didn’t report infections and the disease was able to spread at the same time that health care workers thought the epidemic was slowing down due to the empty hospital beds all around them. When the disease finally got to the point where it was impossible to hide and people started seeking out treatment, it was too late.

Epidemiology is a very cool field of science that often goes unappreciated. There are currently thousands of hard working people putting themselves at risk to contain the situation in Africa, but they need more help. If intervention isn’t stepped up soon, the rate of new cases of ebola is expected to rise to 10,000 per week, because even with an R₀ of 2 the spread is still exponential.  If you are able, please donate to Doctors Without Borders, who are leading the fight against the spread of the disease. Your money won’t only go towards treating the sick, it will buy gloves and masks and proper equipment to help keep those doctors as safe as possible.