Milwaukee County was struck by a rash of fatal drug overdoses this past weekend. Six people died in a 24-hour period.
The medical examiner's office hasn't confirmed whether heroin was to blame. But the drug frequently is, and it has killed hundreds of Wisconsinites in the past few years.
We revisit what we learned in our 2013 series, "In the Grip of Heroin," to examine what makes heroin so dangerous. There are at least three factors.