March 27, 2005


The Marburg virus outbreak in Angola has spread to a new city.

The SARS virus is airborne, though doctors haven't documented any cases that were definitively contracted through this avenue of exposure.

A vaccine is being tested for avian flu. A doctor makes a point at the end of the article that a pandemic is overdue, it's just a matter of when it will hit.

Epidemics tend to hit overly large populations as though, well, like it was some kind of law. Mainly, it's opportunism. Viruses and bacteria have to 'eat' and reproduce like every other living thing. (I will here skip the debate over whether viruses are alive, which I'm in no way qualified to comment on, mostly because I don't care.)

Big, homogenous populations are the most likely to get hit, and subsequently to suffer huge losses, because it's easy to spread from one to another. It's just a matter of what's around to eat, and is a common problem of farmers who must manage their own artificially homogenous populations. Farmers have it even worse, because their populations may in fact be clones, which are a particularly tasty target.

Humans have populated just about every corner or the planet, so they're easy to find. Also, we consume vast quantities of the planet's primary photosynthetic production (plants), whether through clearing vegetation or just eating it, and through this we're starving out and replacing other hosts for bacteria and viruses. The original Ebola cases, for example, are thought to have come from humans pushing into previously undisturbed habitats, offering opportunistic pathogens a new opportunity to jump species. And we often replace a diverse habitat harboring many species, which provides some natural bulwarks against the spread of disease, with monoculture habitat containing few species, which is an open buffet for germs.

Additionally, many members of our species live in filthy or stressful conditions, often in close proximity to other carrier species, and quite a few have too little to eat. All conditions that weaken the body's defenses against parasitic organisms.

So as you can see, we're a large and tasty target for any microorganisms that can evolve some way to consume us. And because there's so little interest, which is to say money, in really improving conditions for those living in poor conditions, we provide a convenient entry point into a globally mobile species.

If public health isn't improved, if humans don't behave in a more intelligent way than bacterial and viral genomes, there will be another global epidemic. Wherever health measures are put in place, where care is made available to at-risk populations, epidemics meet their match and can be made to abate.

So, here's the question: Do you believe that the present government of the US is smarter than the genome of an Escherichia coli?

Update: Clarification thanks to the mad biologist ;)

Posted by natasha at March 27, 2005 03:36 AM | Health/Medicine/Health Care | Technorati links |

Let's just say I'm not betting on the alleged humans being smarter.

Posted by: palamedes at March 27, 2005 07:57 AM

Good post. Just one minor quibble: microorganisms aren't switching to humans because they're running out food, they're successfully jumping to humans because they're getting opportunities to do so:

1)SARS-jumped from civets to humans; too much human-civet contact.

2) HIV/AIDS-simian-human contact-probably by eating bushmeat (genetic data indicate that this isn't necessarily a recent phenomenon)

3) E. coli 0157:H7-changes in slaughtering techniques; we're process cattle too quickly and don't clean the meat well enough. Also, changes in feed seem to influence the prevalence of O157:H7-again this doesn't appear to be a really recent bug.

Posted by: Mike the Mad Biologist at March 27, 2005 08:18 AM


Posted by: Thomas Ware at March 27, 2005 09:46 AM

The really big plagues were in 1400 and 1600, with a big round of flu in 1918. I suspect we are overdue for something really quick and vicious.

Public health? The US is going to be a good target for illness. We will have people who feel they can't afford doctors dying before you can say "Hillary".

Posted by: Scorpio at March 27, 2005 12:49 PM

Mike - In terms of switching, you're right about the proximity, and perhaps I should clarify that. But the fact is that our presence is crowding other species out of their natural habitats, and creating often severe selection pressure.

Posted by: natasha at March 27, 2005 03:52 PM

Actually, as contemptible as Bush/GOP health policies are, I have to disagree with you on this one.

First, the WHO has been very effective, thus far, in stopping epidemics (of SARS, e.g.) by mobilizing response to outbreaks. And the Gates Foundation is pumping a lot of money and expertise into vaccinations/innoculations and sanitation projects in underdeveloped countries. If a real, nasty, lethal/virulent bug starts to burn its way across the world, I'm pretty sure the WHO and Gates will be on it fast.

Second, the more virulent and lethal a virus, the *less* likely it is to go epidemic, much less pandemic, for the simple reason that it kills too quickly and burns itself out before the carriers can infect a lot of people. Ebola is a good example of this: a back-country disease of surpassing lethality that wipes out villages before anyone in them can flee to a city. (Ebola is also, IIRC, not an airborne disease; some kind of direct physical contact is necessary for contagion.)

Third, I strongly disagree that homogenous populations are most vulnerable to epidemic diseases. Au contraire: diseases which one genetic population has lived with for a few hundred years only become really dangerous when exposed to new and different populations that have not adjusted to them. In fact, two of the great plague outbreaks noted above - Black Plague in 1400 and the Spanish flu in 1918 - were a direct result of old germs finding new populations. In 1400, it was the Crusades and expansion of trade that brought 'exotic' microbes from the Mid-East and Asia to Europe; in 1918, it was the movement of WWI troops to exotic locals who became infected and then came back to the US bearing the virus.

Now, once a bug hits a new population, that new population's homogeneity can, yes, be a factor in the spread and lethality - but only because the bug is new unfamiliar to everyone in the same fashion. But the thing that makes a formerly-not terribly dangerous bug a really godawful bug is heterogeneity: many, genetically varied new bodies the bug can mutate in.

Posted by: CaseyL at March 27, 2005 04:02 PM

You make several good points, but...

First, as good as WHO and the Gates Foundation are at stopping these epidemics, no one's perfect. There were raging epidemics in US cities before the advent of public health measures, and there could be again. The periodic outbreaks of TB and West Nile have been contained so far, but further disintegration of care could tip that balance within large US cities, whether the rest of the world manages to stop such epidemics or not. Additionally, we're creating superbugs here by the continued misuse of antibiotics, especially in agriculture. There's no guarantee we won't spawn something nasty ourselves, without even needing migration to spark it, though we continue to be invaded by disease carrying foreign organisms that will continue to flourish better as the climate warms.

Second, I don't think I did say that Ebola was airborne, that's SARS. Yes, extremely lethal diseases may wear themselves out quickly, but they may also develop less lethal strains given such selection pressures. Syphilis used to be exceptionally lethal, and a far faster acting bug, but the strain that's persisted into the modern day has become a pale shadow of its virulent ancestor. Given the repeated outbreaks of hemorrhagic fever, though they've admittedly not been increasing as rapidly as many less lethal diseases, it isn't improbable to suppose that a strain with delayed or less lethal reactions might develop as human exposure continues.

Third, you're right, but not in such a way that mutually excludes the problems of homogeneity. In terms of agricultural disease, homogeneity is every bit the problem I described. In terms of human experience, the Native American population is thought likely to have succumbed so quickly to the new diseases brought by explorers because their immune systems were very homogenous compared to the plague-wracked ancestors of the Europeans. And we're now giving old viruses many more chances to confront new populations, pushing progressively into the habitats of their old hosts.

I'll grant you though, without argument, that the topic is too complex and multifaceted to write a few simple paragraphs in a blog post that convey the whole truth. You get a little here, and a little there, some of which sounds contradictory taken in isolation. I'm not trying to portray the whole truth, nor can I, so I'm glad when others fill in other aspects of the picture.

Posted by: natasha at March 27, 2005 04:41 PM

Very good points, all.

Oh, I didn't mean to imply that you had said Ebola was an airborne pathogen; I was just pointing out that as another reason why there haven't yet been any Ebola epidemics.

And thank you for bringing up syphilis. The history of syphilis was the first time a disease etiology caught my attention, back when I was but a teen. And *that* was because I'm also a history geek, and wondered why Henry VIII's syphilis was so different from the variety we learned about in sex ed.

Another factor in epidemiology that doesn't, SFAIK, get much study is how and why pharmaceutical companies seem to have abandoned research into cures for diseases in favor of research into treatments. Is this is a profit model thing (someone with a really mordant sense of humor once said the pharmas must be kicking themselves for coming up with a successful vaccination against polio, that only needs to be administered once, rather than a palliative, that people would have to buy for the rest of their lives)? Or is it due to a fundamental difference in the nature of the intractible diseases we're still dealing with, e.g., retroviruses and autoimmune reactions rather than microbial? It would be interesting to know.

Posted by: CaseyL at March 27, 2005 05:31 PM

The first I read about syphilis was in reading an account of Talleyrand's life, and when I later put that together with its current incarnation, I had about the same reaction as you. It's fascinating stuff.

I suspect you're right that it is a profit motive thing, though. TB is a huge killer around the world, but the people who have it now are the same populations who already can't afford AIDS treatments. Private enterprise is great, but sometimes society needs to get things done that aren't directly or immediately profitable, and governments are better at that. A lot is being spent on AIDS research, but enough of the victims are wealthy westerners that it pays to support it.

And in terms of getting rid of these diseases, a lot of prevention mechanisms could be undertaken that could stall or halt their spread. Even if we didn't manage to cure many of the diseases spreading through at-risk populations, much, much more could be done to stop new cases without a single new bit of research. We already have condoms, mosquito netting, water treatment, public sanitation, and knowledge of what constitutes decent basic nutrition sufficient to support normal immune function.

I'd be ecstatic if we'd just do more of what we already know to work, even if it meant diverting money from certain research. But I think we'd save so much money from prevention that it would pay for itself, and it wouldn't have to be an either/or decision in the long run.

Posted by: natasha at March 27, 2005 06:38 PM