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.

Sketchy Fact #49: Small-Pox Won't Cause Pandamonium in Animals

Smallpox only infects humans and is not carried by any wild animals. It is the only natural biosafety level IV infectious disease (maximally dangerous) not to have an animal host.

Sketchy Fact #47: World's Deadliest Virus

Until 2004 when an experimental treatment saved the life of Jeanna Geise, rabies had a 100% fatality rate. It remains the deadliest known virus on the planet once an infected person shows symptoms.

Infecting with Immunity: How Vaccines Actually Work

One of the major challenges scientists face is communicating important information to the general public. As brilliant as many of them are, hours spent in a lab hunched over a microscope do little to improve a person’s ability to explain things simply and comprehensibly. Complicating things further is the fact that scientists are trained to constantly question and test their ideas so they say a lot of things like “we believe” and “these results suggest” when really they mean “I am sure this is true, these are facts.” When you’re talking about the evolutionary history of Amazonian frogs or something like that, it tends not to matter; but occasionally a topic comes along where a clear explanation of how things work is even more important than a detailed reporting of the facts. Vaccines are one of those topics.

Right now in the Fraser Valley of British Columbia Canada, several hundred people (mostly children) are infected with an unpleasant and occasionally fatal illness that scientists basically cured a long time ago. Over 200 cased of highly contagious measles have popped up in the region and a few infected people have found their way to Ontario and the Atlantic coast, largely because the Fraser Valley has one of the lowest measles vaccination rates of anywhere in Canada (between 70 and 80%).

Much of the problem lies in the fact that vaccines inject viruses and bacteria into people’s bodies. Generally speaking we are taught that this is a bad thing, so our instinctive reaction is to avoid it. Teaching people that infections are bad and then telling them that you need to inject them with a virus is generally a hard idea to sell because, quite frankly, it makes you seem insane.

The truth is vaccines actually are made of the viruses and bacteria that they are trying to prevent. To understand why that is necessary we need to understand the immune system. You see, when an unfamiliar virus or bacteria makes its way into your body it sounds an alarm that activates an immune system response. These invading organisms are called antigens and their goal is to replicate as much as possible. 

Since our bodies are not designed to handle that sort of thing the results can be very bad, resulting in sickness and sometimes even death.

Your body doesn’t like dying, so once that alarm is sounded you’re white blood cells get to work. Your T-cells identify which cells in your body are infected by the intruder and destroy then to try to contain the infection. Meanwhile your B-cells produce antibodies, which attack antigens directly and try to prevent them from infecting more of your body’s cells.  

Vaccines take advantage of this response by tricking your body into thinking it has been infected by a specific virus or bacteria, but in order to do that they need to introduce some of these intruders into your body. Obviously the goal of vaccines is to prevent infection so it wouldn’t make much sense to pump you full of measles. Instead, vaccines use a weakened or inactive (dead) form of the intruder/antigen.

Vaccines that use weakened antigens are called live-attenuated vaccines, because they inject still living infections. The trick with live-attenuated vaccines, however, is that they use the dumbest, laziest individual viruses and bacteria possible. To create these vaccines scientists take something dangerous like measles and give it a relaxed, cushy life in a test-tube. The infection is continually transferred from one test tube to another, each time getting a little more used to the good life. After enough time and transfers (77 in the case of the measles virus) the infection is so used to not have to do anything to survive that it has lost most of it’s tenacity. When injected into the bloodstream these infections reproduce at most 20 times rather than the thousands and thousands of times they normally would. Your body is easily able to identify and destroy these antigens.

The cool thing is that you immune system has a great memory, so once the infection is gone your B-cells continue to produce the antibodies that can kill it, making you immune for life.

Inactivated vaccines use dead viruses that don’t reproduce at all. The benefit to this is that there is zero chance of any kind of negative reaction (live-attenuated vaccines can lead to soreness at the injection site and occasionally some mild symptoms), the downside is that you often need multiple shots to maintain immunity because your body thinks the disease is a dud.

One of the most publicized fears about vaccines is so-called link between vaccination and autism. This link is a myth. It arose when some people discovered that a disinfectant used in some vaccines (Thimerosol) contained mercury. There is however no reliable research supporting this claim (Parker et al., 2004). Even still, most vaccines now contain no Thimerosol because the companies that produce the treatments want to make parents feel as safe as possible. One real health concern that goes along this vaccines is the risk that some flu shots pose to people with egg allergies. Since influenza is grownin eggs to create the vaccine, there is a chance it can cause an allergic reaction. If you’re allergic to eggs to can still get a flu shot, you just have to talk to your doctor about getting one of the varieties that is not developed using eggs (there are plenty).

Vaccines are clearly a very cool topic. They have eliminated diseases like polio and smallpox from much of the world and continue to save millions of lives each year. In closing, I just want to say that it is okay to question things. Skepticism is a great quality that can lead to critical thinking and amazing ideas and innovations. That being said, the science behind vaccines is inarguable. They prevent needless suffering and protect at-risk people from terrible diseases. If you have questions about them, by all means continue to read up on the subject; but rest assured, vaccinating yourself and your children is the most responsible decision to can possibly make. It’s a no-brainer.