Hi, Mark Armon, this is a common English idiom. "Take a turn for the worse" means to start to get worse. Example: "It appeared that she was going to get well; then, unfortunately, she took a turn for the worse." Another example: "My job was going quite well; then last week things took a turn for the worse."
It basically means that something was going well, but it suddenly started to get worse.
I hope this helped!
-Kyle