(http://www.randalolson.com/wp-content/uploads/wsj-polio-dataviz.png)

I find this visualization extremely interesting, but also possibly confusing. I’m going off the assumption that the colors are representative of the number of people with Polio. I wonder how some places managed to go from a very dark orange back to blue/green. Did people just die so therefore the number of people with polio decreased? Or am I wrong to assume the color scheme?