Do Helmets Prevent Head Injuries?

A cycling enthusiast asked me about helmets. It seems compellingly obvious to me that a head impacting the pavement without a helmet is likely to sustain more damage than a head protected by a helmet. He challenged that, citing a BMJ article by Ben Goldacre that questioned whether the evidence showed that helmets do any good. He said I

Worms, Germs, and Dirt: What Can They Teach Us About Allergies and Autoimmune Diseases?

Humans evolved in an environment where they were exposed to animals, dirt, and a variety of pathogens and parasites. Our immune systems evolved to cope with that environment. Now most of us live in a different environment, with safe drinking water, flush toilets, food inspection, immunizations, and public sanitation. This means that we are far