Flu activity is labeled on the map from "minimal" to "intense" based not on reported cases but on the number of flu related Google searches conducted in the area. Because the map doesn't report on actual cases, it's not a perfect scientific representation of worldwide flu activity, but the logic behind the project is interesting. Because Google collects and analyzes this data in real time increased search activity could be used as a signal to health officials that a flu outbreak is coming.
It's a little freaky to think about — Google will know a breakout of the flu is coming even before we know it's coming (or before doctors, for that matter), but I love the idea behind it. In fact, literally hours before I saw the site, I typed "flu symptoms" into Google. You can read more about the project's details on the FAQ page.