Why are kids always sick? Stress research in primates might point to an answer.

Most parents will know what I'm talking about when I say, in response when people ask how my son is, "He's between colds." That's the answer I give if he's not actually sick, either with a cold or some random virus that doctors just shrug at and say he seems to be fighting it off okay.

Kids are always sick. This seems to be a fact of life, at least life in the Western world, which is where most of my experience is limited to. Whenever I take John to a toddler group, or invite people with kids over for dinner, it's almost guaranteed that he will come down with something about 24 to 48 hours later.

It seems like a remarkably stupid decision of evolution (like teething, also designed poorly, as it drives everyone to distraction and keeps me, at least, from wholeheartedly fulfilling my son's needs) that I generally come down with exactly what he has at just the time he most needs me to be fully functional.

(Pause while I read John the soft piggy book five times in a row, and then wipe the accumulated snot off his face.)

The question is, why? I was talking about this with my sister a few weeks ago. We all take our children's constant minor and exasperating illnesses as a matter of course, but it suddenly struck me as very odd. So I've been asking everyone I know -- do humans actually get sick a great deal more than other animals? And if we do, it really leaves you wondering not only why, but how on earth we've survived this long.

Nobody seems to have an answer, although I've come across one possible explanation. (The surprising part about this unscientific survey is that the question doesn't seem to have occurred to many people, which tells you something about how mentally exhausted most otherwise intelligent parents are.)

In a 2007 article on sciencedaily.com Robert Sapolsky, a neuroscientist, discusses his decades-long research on the social behavior of primates, and the greater incidence of stress-related diseases among primates and humans. His words put it best: "Primates are super smart and organized just enough to devote their free time to being miserable to each other and stressing each other out," he said. "But if you get chronically, psychosocially stressed, you're going to compromise your health. So, essentially, we've evolved to be smart enough to make ourselves sick."

Higher stress levels are certainly a factor in reduced immune system function, which could explain why I've spent the last two days blowing my own nose as well as wiping my son's, although I don't think it gets into the issue of why human children get sick so frequently in the first place. Sapolsky's research, at least in this article, focuses more on stress-related illnesses such as heart disease and diabetes.

So I'm still asking the question: why the heck is my kid's best health simply "he's between colds"?