There's a sad irony in that rule. Though models must fit into a size zero to walk the runway, the CFDA stipulates that models who are identified as having an eating disorder must seek professional treatment and stop modeling until that professional clears them for the industry again.
The discussion of too-thin models began in earnest in 2006, when Brazilian model Ana Carolina Reston died of a generalized infection due to anorexia. Her death caused the organizers of Madrid Fashion Week to institute a minimum Body Mass Index (BMI) of 18.
According to The Associated Press, at the time of Reston's death, she weighed 88 lbs., which put her BMI at 13.4.