Make that germs: abstract here, other review here, older review of previous work here (1st link via Jerry Pournelle). Even when food intake & activity was controlled, chickens became obese when infected with a human adenovirus (Ad-37, though other adenoviruses were already known to be implicated). Presumably they play a role in human obesity as well, and so yet another case where germs play a role in chronic illness, as predicted by Gregory Cochran & Paul Ewald's New Germ Theory (click 1st item), which Ewald popularized in Plague Time. Of course, germs aren't the only cause of obesity, since there are plenty of ways to screw up a system. Aside from usual suspects like poor diet & exercise, I thought up another obesity germ scenario here (item 3i). Exploring this study further:

1) These viruses are respiratory & spread by droplets, so this may be a case like polio where a germ occasionally meanders from its natural habitat (for polio, the gut) and does inadvertent damage elsewhere (for polio, the nervous system). So, implication of germs in an illness at site X doesn't imply that the germs are adapted to exploit site X, and thus we don't need to spin an evolutionary story about how the germs screwing up X helps them spread. Polio only infects the nervous system about 1 per 100 cases, the rest of the time hanging out in the gut.

2) Also like polio & other known infectious diseases, obesity could turn out to be simple to cure once a vaccine were engineered. You don't cure dysentery with surgery or GE -- you just clean up the damn water. With all of the health care costs due to obesity & its concomitant health problems gone, it would be a hell of a financial relief for all.

3) Speaking of Behavior Genetics below, studies like these highlight the role that microbes play in individual differences, namely the mysterious "non-shared environment" which accounts for ~50% of the variation in intelligence & personality. That is, one monozygotic twin gets infected, the other doesn't; both get infected, but the germ deviates from its niche to some unintended site in one but not the other; both get infected, the germ deviates from its niche in both, but one twin houses some other germ that blocks the path of the deviant one before it can get into the nervous system.

Emulating obesity researchers, we need a hard-nosed approach to figure out the "ideal" range for Big Five personality traits, as nutritionists have for BMI. That is, not "ideal" for living an enjoyable, fulfilling life, but "ideal" in the biologist's sense of optimal for making babies. Suppose investigators find out that some population's mean for, say, Introversion is a standard deviation or so below the ideal score -- as if the mean for BMI were far above ideal. Now, if some individuals were below the ideal, no big deal, but if the population mean were noticeably below the ideal, it would suggest either environmental toxins or pathogens -- or selected genetic response to such pathogens, like sickle-cell.

After all, if germs are implicated in obvious mental illnesses such as schizophrenia (considering its frequency, fitness cost, and having been around for at least several hundred years), these illnesses may represent only the extreme cases of a more general pattern of microbes negatively affecting cognition (vis-a-vis reproductive fitness), much as pneumonia represents an extreme complication arising from typically mild influenza.

Labels: disease