Where diseases come from

There's an interesting review article on infectious diseases in today's issue of Nature (May 17, 2007). It's titled "Origins of Major Human Infectious Diseases" and authored by 3 researchers at UCLA, Nathan Wolfe, Claire Dunavan and Jared Diamond (author of Guns, Germs, and Steel).

There have always been opportunistic diseases of one sort or another. Even bacteria (germs) can get "sick" from viral infiltration. However humans have artificially created a situation today in which they get sick a lot more than they did in the distant past. You see, since the advent of agriculture around 10,000 years ago humans have maintained large stationary populations and also segments of the human population have stayed in direct contact with populations of other animals (chickens, goats, cows, dogs, etc). Both of these trends have culminated in a number of diseases that would not have occurred in hunter/gatherer societies.

This article explores the evolutionary stages of how pathogens affect us--delineated into 5 stages--and by evaluating those 5 stages the authors reach conclusions about the history of human society. How disease has affected population movements as well as the dangers of pathogens in our current world.

Figure 1 in the paper illustrates quite succinctly how pathogens get introduced to humans and how the pathogen can change to adapt itself to the human host--or doesn't do so. Some diseases never move past stage 1, which is characterised by being solely passed from an animal to a human, rabies being the example given. On the other end of the spectrum is stage 5 in which a disease is only passed from human to human and animals are never involved. HIV-1 is the example given here.

A topical warning is reached in that the animal reservoirs for disease are more prevalent in the tropics--so our increasing incursions into the tropical rain forests increase our chances of releasing such a pathogen into human populations. Yet, in seeming contradiction to that, most of the diseases that bedevil humans had their origin in the relatively cool temperate cities of Europe. This is attributed to 13 out of the 14 major domesticated species are native to Europe and Asia. The sole domestic species native to the New World is the llama.

Today we're staring down the barrel of an epidemiological gun with Mad Cow Disease and SARS possibly poised to rip through our population. This article is a good primer for just how serious our risk is and what the future might hold for us. The worst aspect is that the authors admit that even some of the best studied human diseases don't have known origins. It's difficult to plan for the future when the past isn't yet known.