Angry Birds (video game) was being played, the staff liked the characters, and the team elected to design a game around them. In early 2009, physics games, such as "Crush the Castle", were popular flash-based web games, so the Rovio team was inspired by these games. During the development of "Angry Birds", the staff realized the birds needed an enemy. At the time, the "swine flu" epidemic was in the news, so the staff made the birds' enemies pigs. Angry Birds was the studio's 52nd produced game and on its initial release, the game did not sell many copies. After Angry Birds was a
Angry Birds (video game) spot on the US App Store a few weeks later, where it remained until October 2010. Since release, the free, limited version of "Angry Birds" has been downloaded more than 11 million times for Apple's iOS, and the full-featured paid version has been downloaded nearly 7 million times as of September 2010. The Android version of the game was downloaded more than 1 million times within the first 24 hours of release, even though the site crashed at one point due to the load, and over 2 million downloads in its first weekend. Rovio receives approximately US$1 million per month