Reply #12: And if an illness is genetic, then an increase in that illness means

[bananas]

something in the environment is responsible.

Here's another quote from the article:

In other words, many kids are affected by pressure-cooker school environments and a culture of thinness promoted by magazines and music videos, but most of them don't secretly scrape their dinner into the garbage. The environment "pulls the trigger," says Cynthia Bulik, director of the eating-disorder program at the University of North Carolina at Chapel Hill. But it's a child's latent vulnerabilities that "load the gun."

So the illness is caused by genes + environment. The article also says the illness is dramatically increasing in younger children, so either:
a) the environment is "pulling the trigger" more often, or
b) there are more children with those genes - which means something in the environment is selecting for that gene. For example, maybe the parents were influenced by the media in selecting their mates.