There is a nationwide recall of toxic poppy seeds. What is thebaine? And how do you get sick from poppy seeds?

A nationwide recall of poppy seeds has been announced after a spate of poisonings across the country. 

Non-food-grade poppy seeds, containing high amounts of the controlled drug thebaine, have since been discovered in the food supply chain. Here's the situation as it stands. 

What's happened so far?