New York's subway system, already among the busiest in the world, carries more commuters than previously thought: researchers have identified 562 species of bacteria in the city's subway stations including 67 species associated with illness, according to The Wall Street Journal.

In 18 months of scouring the system, Weill Cornell Medical College geneticist Dr. Christopher Mason and a team of researchers found germs that can cause bubonic plague, meningitis, stomach trouble and antibiotic-resistant infections. They also found bacteria that keep the city livable by sopping up hazardous chemicals or digesting toxic waste.