So, you basically match the colors of the pills with the colors of the viruses or germs. Eliminate all the viruses in the bottle as quickly as you can before moving on to the next level. Pretty basic, but challenging nonetheless
Like in Yoshi's Cookie, Mario was the baker who controlled the cookies, and now in his MD coat, Mario controls the pills to kill the germs. It brings back memories, personally, and from what I remember, it was fun. There is another Dr. Mario game for the N64, and the game has also appeared on the SNES, the GBA, Wii, and even the game 'Brain Age 2' for the DS. So buy it if you have a NES, because this will certainly be a rare game someday, among hundreds of others.