You're making a mistake. Apps have a fixed life-cycle. Once everyone who wants your app has your app (happens quicker with niche products), then it will start dropping in ranks.
If you look at the top 10 apps in the medical section (or the top 25 for that matter), you'll see that they have been there for months (or years) on end.
My feeling is that the medical section of the app store is fairly different from the games section (where apps have a very limited shelf life).