In case you missed it, over the past couple of days there have been reports of an outbreak of Ebola hemorrhagic fever virus in Uganda. As of this writing, the most recent report I’ve seen puts the death toll at 16, with a few other suspected cases. Ebola is terrifying for a number of reasons – it’s readily transmissible, it has a remarkably high and rapid lethality (25-90% case fatality rate within days to weeks), and the way it kills is gruesome – causing massive bleeding from all orifices. There’s no vaccine or cure.
The good news from an epidemiology standpoint though, is that Ebola is a kinda terrible pathogen in humans. It has a short incubation time, and infected people are obvious (what with the bleeding from their eyes). Further, it’s not airborne, so you actually have to come into contact with bodily fluids from infected individuals to become infected yourself. Once an outbreak has been identified, health workers can take fairly simple precautions and dramatically reduce their risk of infection. What this means in a practical sense is that, after the first terrifying days of lots of sick people turning up, people know what to do to avoid infection, and the outbreak burns itself out. Since its discovery in 1976, Ebola has killed less than 2000 people, with the worst outbreak in 2000 killing 224. Contrast that with your garden variety influenza, which kills on average 40,000 people per year in the US alone, and hundreds of thousands more around the globe.
Remember, the goal of a pathogen is not to kill you. Your death is an unintended side-effect of the real goal: replication and transmission. On this front, Ebola sucks. It’s great at evading your immune system and replicating like crazy, but then it makes you terrifying to other potential hosts and incapacitates or kills you before you have a chance to spread it very far. The most successful pathogens strike a balance between replication and transmission. Influenza generally leaves you healthy enough to walk around and shake hands, press the elevator button at work, and use an ATM. Influenza doesn’t want you dead – the only reason it kills so many people is that so many more people become infected. Successful pathogens that make most of their victims really really sick or dead have more efficient ways to be transmitted that don’t require you to be healthy and mobile. Malaria has mosquitoes do the work of getting the infection mobile, cholera gives folks massive diarrhea to spread itself in water sources, Bacillus anthracis (anthrax) can form spores that survive time, heat and desiccation for decades while waiting for a new host.
So, Ebola is not really as scary as it seems. The really scary thing about this outbreak and others is that it reminds us that brand new diseases come out of previously isolated areas, and that globalization and urbanization means that the distance between a rural village in Africa and every major city on the planet is small and shrinking. This most recent outbreak of Ebola had victims in Kampala – the capital of Uganda. It’s fairly easy to imagine an international traveler hoping a plane from Kampala to Paris and then… Well, Ebola probably wouldn’t get much further for the reasons I already mentioned, but some other virus?
New infectious diseases generally pass into humans from animals, but they’re poorly adapted for our immune systems. Generally, this means that they either don’t replicate very well, or are difficult to transmit, or they’re super deadly and wipe out their hosts too quickly to be transmitted. The trouble with cities is that high population density lowers the bar on transmission, allowing more virulent bugs to make it to the big time. And rapid global transit means that any local outbreak has the potential to cause a global pandemic.