The beasts inside us
About 90% of you isn't actually "you". In terms of cell count, a typical human being is outnumbered roughly 10 to one by the microbes that reside in and on their body. You and the microscopic beasts inside you are a complex ecology; mess about with one part, and unexpected things might happen.
When we take antibiotics, we're hammering that ecology, sometimes not for the long-term good of us or the beasts.
There was a truly remarkable article on just this topic in Nature a couple of months ago: "Stop the killing of beneficial bacteria" by Martin Blaser of New York University (Nature 476 pp 393-4; see also article about it in the New York Times.).
He points out the obvious; every time we give antibiotics to a child or an adult, we change the bacterial community vastly.
He gives an interesting example: virtually everyone, he claims, used to have Helicobacter pylori in their stomachs, and these H.pylori are blamed for gastric ulcers and some stomach cancers in later life.
H.pylori and humans have lived together in the same body for some 58K years, or so Blaser says. Now, in Western societies, only few people have them.
Yet mice with H.pylori when young have less asthma, when 'cured' of H.pylori (i.e. dosed with a specific antibiotic that kills 'em off) they get asthmatic.
Blaser blames our vast increase in asthmatic diseases on our present lack of H.pylori! That is - admittedly - speculative. But there's no doubt that he's right about the impact of antibiotics in general.
There are lots of ways in which the beasts inside us are different from those inside the bodies of our forebears. Modern sanitation, more hygienic practices across a range of human activities notably in food production and smaller families all play a part in changing the 90% of "us" which isn't actually us. We are only just beginning to understand how our immune systems, in particular - and our digestive systems, in general - are interactive with all those beasts inside us.
Intriguingly, too, research on obese people shows they have different bacterial ecologies from those of slim people. The US National Institute of Health has funded Blaser some $6.5M grant in 2010 to investigate the disappearing microbiota in the current obesity epidemic.
This supplements the $115M grant NIH gave in 2008 to the Human Microbiome Project. This is potentially as big a deal as the Human Genome Project; it is to identify microbes that reside on and within a healthy human being -- we await their findings with great interest.