Apologies, dude. But as much as you fear purse cooties, it's impossible to escape germs. While millions of riders are jostling daily for seats, far more microbial organisms (bacteria, archaea, fungi) are duking it out on the subway's surfaces, says Christopher Mason, a geneticist at Weill Cornell Medical College. Germs do make us sick, but it's increasingly obvious that our health also depends upon the microbes living inside our bodies (the human microbiome), and in our built environment. While subways are a defining characteristic of the daily commute for people worldwide, little is known about how they shape the life of germs.