A pandemic is a serious outbreak, and kills many people. For example, in our current flu outbreak, over a dozen have died, and counting.
Medicine prices rise, as supply and demand comes in to full affect here. Lots of people want medicine, and they go out of stock in the blink of an eye. Additionally, people get flu shots to protect themselves, but it's possible the strain isn't very protective. The topic is debatable in modern day.
During all this, the virus, like any other - evolves. It mutates and starts being able to fight off medications and the immune system, new medicines are made, and the whole cycle starts again.
