Questions

1) If they use the same cutting board/counter to cut raw chicken, and other foods with out sanitizing in between. 

2) If they have a cold, flu, or pink eye and are handling food, coughing without covering their mouth, then washing their hands.

3) If they don't properly wash their vegetables.

4) Not storing foods at proper temperatures.

And so many other reasons for Food handlers can contaminate food.

The long-run aggregate supply curve is vertical because it is at the full-employment or potential output. Because the economy's potential output is determined by the availability and productivity of real resources, not by the price level. That means that even if demand increases, firms can't hire new workers and expand because everyone is already working.