Angry Birds is a viral hit. A virus everyone wants. All images courtesy of Rovio.

When it comes to computers there are two kinds of viruses: the ones you don’t want anywhere near your machine (but seem to sneak in there anyway), and the ones you seek out and willingly add because everyone says they’re great.

Angry Birds is an example of the latter: a cheap or free (depending on whether you have an iPhone or an Android) and addictive downloadable game for your mobile device. Since its introduction in 2009 Angry Birds has gone viral, big time; you might say the game is a virtual Bird Flu – but far more benign and a lot more fun.

In case you haven’t played the game (or stood in line behind someone who was), Angry Birds is an ongoing battle of the species, as the aforementioned avians wreak revenge on the swine who’ve swiped their precious eggs. Their method of combat: launching themselves via slingshot at various pig redoubts, with the goal of bringing the oinkers’ fortifications tumbling down on their chubby heads. (In fact, pig and bird alike are body-free, consisting only of heads, which might explain why the wingless birds need a slingshot to attack their enemy.) That such attacks might also crush their captive offspring evidently never occurred to their bird brains.

A good thing it didn’t; if it had, the game’s millions of fans would have missed out on a lot of hand-held entertainment. Angry Birds was first hatched in the mind of Jaakko Iisalo, senior designer at Finland’s Rovio Mobile, a company specializing in games for the mobile market, in the form of a single concept drawing:

“We were going through various proposals for game designs,” recalled Rovio’s CEO Mikael Hed in a 2010 interview. “One of these was a mock screenshot that had some really angry looking round bird characters with no wings and no legs. And they were trudging along the ground with a cloud of dust trailing behind them…We didn't really understand very much about what the game mechanics were like there, but everybody in the room really liked the bird characters. So we set out to design a game around those birds.”

100,000 Euros worth of R&D later, an iPhone-only version of Angry Birds went on sale in Apple’s App Store for the pocket-change price of 99 cents. (Later versions and add-ons cost more.) This first version of the game consisted of three chapters, each containing 21 levels of increasing difficulty players had to defeat before moving onto the next. In each level the goal is the same: slingshot a bird at the pigs’ base, hit its weak point and send the structure tumbling to the ground. As the game progressed, new birds with different abilities appeared, the pigs’ structures grew more elaborate and resistant to attack, and that sweet spot became harder to find – or hit.