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Dear Mark: The Origin of Human Infectious Disease

Posted By Mark Sisson On November 9, 2010 @ 9:29 am In Dear Mark,Health,Research Analysis | 29 Comments

Just a light and playful topic for your Tuesday morning…. What can I say? Since my posts on human longevity, I’ve had germs on the mind. Aaron Blaisdell’s response [7] to Part 2, however, truly inspired today’s topic:

Excellent post! I wonder how much infection afflicted human populations prior to the adoption of animal domestication. Jared Diamond has discussed how animal domestication led to an increased transfer of viral infections (e.g., colds, flu, small pox, TB, etc.) from farm critter to human. Prior to the domestication of animals such viral transfers were probably rare. Death from viral infection may have been much lower in pre-pastoral times.

So, what kind of infectious landscape did our paleo hunter-gatherer ancestors inhabit? Did their living conditions do more to imperil or spare them? And how did their chances change once they acquired agriculture and animal husbandry? How does it compare to the picture in our modern age?

To understand Grok [8]’s exposure to infectious disease, we can look to Grok’s lifestyle, his interaction with the environment, and his social organization. Grok’s kind, of course, lived in small, nomadic bands that had little interaction with other groups. They killed and ate animals but didn’t keep them. They didn’t cultivate or significantly alter the land they lived on. Although disease existed on some level, broad exposure to a large number of diseases wasn’t what it became [9] later when humans settled into larger, stationary communities.

That all changed when humans took up agriculture and animal husbandry. These novel practices allowed the human population to increase but the added numbers and density of settled communities created the perfect scenario for certain disease transmission [10] (a.k.a. “crowd diseases). Add to this picture the introduction of livestock animals, and suddenly you have a whole new infectious potential.

Experts generally agree that animal husbandry had a hand in introducing new diseases to the human population. Human “crowd diseases” are evolutionary derivatives of animal pathogens as molecular studies have shown [10]. Nonetheless, the picture is more complex than it first appears. As one review of the research [11] suggests, the ultimate catalyst for the boost in pathogen transmission to the human population appears to have been the ecological changes related to animal husbandry and related agricultural practices. In other words, it wasn’t just the increased proximity to animals themselves but the way our farming and herding ancestors manipulated the immediate environment to support their livestock and additional crops. The way they modified the land for their use altered the “transmission ecology” for disease. Issues regarding animal and human waste, food and water sources likely figured into the equation. Changes in the environment and the introduction of livestock also altered these humans’ indirect interaction with wildlife. In some parts of the world and in the case of some diseases, interaction with wildlife was the more primary threat [12] and remains so to this day.

Today, approximately 60% of new human pathogens are acquired from animal sources, and more than 70% of those are based in wildlife [13]. With our species’ continuing movement into new wild habitats, we expose ourselves to the unique pathogens that exist there. Just as our agricultural ancestors interacted and manipulated the land in new ways, we continue to do so today. Humanity’s pathogenic web keeps growing.

Our modern infectious landscape, of course, is something akin to a stepped up version the Kevin Bacon game. With our penchant for international travel, there aren’t very many steps (or hours really) between an outbreak in one part of the world and its introduction here. The Journal of Travel Medicine just recently published the results of a survey [14] showing that some 38% of international travelers from Boston’s Logan Airport were traveling to low and middle income countries, where infectious disease is typically more common and less monitored. Of that 38%, an unnerving 46% didn’t bother to learn about serious infectious risks – like malaria, typhoid, hepatitis, and dengue fever – common to their destinations or what they could do to help prevent contracting these infections. That probably doesn’t bode too well for the rest of us.

Closer to home, we deal more often with the likes of E. coli and MRSA from hospital transmission and modern agricultural practices. As if E. coli in beef and salmonella in chicken weren’t enough, now we’re seeing [15] MRSA in pork [16]. (I guess you can add bacon to the Kevin Bacon game.) Although we see less of the tropical diseases that plague much of the rest of the world, diseases like TB are on the rise, and we now have million people living with HIV/AIDS [17]. For more information on infectious disease numbers and policy in the U.S., check out the Kaiser Foundation’s website [18] for a truly impressive (although sobering) array of interactive maps, reports, and comparative graphs.

These days we’re all too accustomed to thinking about massive pandemics. Some go underreported for their impact and others get hyped far beyond the legitimate toll they take. (H1N1 [19], anyone?) Media field days aside, we hold both historic and recent scourges in our collective consciousness: the Bubonic Plague, Spanish influenza, TB, AIDS, cholera, malaria, Ebola [20]. (Light reading as promised…) Just as growing population density put our ancestors at increased risk, the same trend holds true today. Add to the population angle our continuing settlement into deeper wild habitats and the modern – yet decidedly unsanitary – livestock conditions that put farmers, handlers, and eventual diners at risk. As a result of these and other influences, the World Health Organization warns [21] we humans have been “gaining” a disease a year on average since the 1970s. Not the kind of progress we thought we were signing up for… To boot, we’re exposed to new and virulent disease while medical overprescribing and agricultural exploitation hampers the effectiveness of the antibiotic and antiviral drug armory designed to protect us. Hmmm.

Modern living, as I’ve always said, is an exchange of risk for reward. The benefits we embrace aren’t free. (In fact, they aren’t cheap either, as WHO’s financial breakdown of recent epidemics shows [21]. The best infectious disease prevention includes part general health, part common sense, part sanitation, and part education. Although, I’m not a fan of alarmists who declare that the sky and accompanying bird flu-laced droppings will fall any minute, and I’m hardly one to suggest that people don’t travel where they want to travel. Infectious disease to some degree has always been with us and is something to control rather than hope to thoroughly eliminate. Nonetheless, I believe it’s prudent to educate ourselves and scrutinize the practices that contribute to its spread. From my perspective, investing in good overall health is a place each of us can start. Choosing – when possible – not to support farms/facilities that use routine antibiotics for their livestock is yet another option. Our age is a far cry from Grok’s, but there’s plenty of room for common sense on the path to progress.

Thanks for reading today, and be sure to let me know your thoughts. Have a great week, everyone!


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[7] Aaron Blaisdell’s response: http://www.marksdailyapple.com/life-expectancy-hunter-gatherer/#comment-667822

[8] Grok: http://www.marksdailyapple.com/about-2/who-is-grok/

[9] broad exposure to a large number of diseases wasn’t what it became: http://www.ncbi.nlm.nih.gov/pubmed/18033595

[10] created the perfect scenario for certain disease transmission: http://www.nature.com/nature/journal/v418/n6898/full/nature01019.html

[11] review of the research: http://www.ncbi.nlm.nih.gov/pubmed/16672105

[12] interaction with wildlife was the more primary threat: http://www.nature.com/nature/journal/v447/n7142/full/nature05775.html

[13] more than 70% of those are based in wildlife: http://www.ncbi.nlm.nih.gov/pubmed/20031035

[14] survey: http://onlinelibrary.wiley.com/doi/10.1111/j.1708-8305.2010.00457.x/abstract

[15] seeing: http://www.vetsweb.com/pigs/husbandry/on-farm-hygiene/mrsa-in-pigs-%E2%80%93-where-are-we-today%3F-1390.html

[16] MRSA in pork: http://www.nytimes.com/2009/03/15/opinion/15kristof.html?_r=1

[17] we now have million people living with HIV/AIDS: http://www.cdc.gov/hiv/topics/surveillance/basic.htm#hivest

[18] Kaiser Foundation’s website: http://globalhealth.kff.org/

[19] H1N1: http://www.marksdailyapple.com/how-not-to-get-swine-flu/

[20] Bubonic Plague, Spanish influenza, TB, AIDS, cholera, malaria, Ebola: http://www.kff.org/globalhealth/guide/global.cfm

[21] World Health Organization warns: http://www.who.int/whr/2007/en/index.html

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