Addiction is a complex phenomenon. Genetic, physiological, cultural and socioeconomic factors all appear to play a role. Now, new research in rats shows that heroin addiction activates brain circuits associated with negative emotional learning, which in turn creates persistent unpleasant emotions that a user suppresses with yet more opioids. 
James Cagney might call these "dirty, double-crossing rats," but they're likely not dirty, and they don't mean to do it. But according to the CDC, pet rats can carry and pass along a type of hantavirus – the Seoul virus – that can make their owners pretty sick.
After five years of living in New York, I have gotten used to just about everything this city brings. With one exception: rats. New research on the genetic diversity of these true New Yorkers shows that they tend to stay local. In fact, it's rare that they even move off of the block where they were born.