The first game of the Halo franchise, it was released on November 15, 2001, as a launch title for the Xbox gaming system, and is considered the platform's "killer app". More than five million copies were sold worldwide by November 2005. Microsoft released versions of the game for Microsoft Windows and Mac OS X in 2003, and the surrounding storyline was adapted and elaborated into a series of novels, comic books, and live-action web series. Players battle various aliens as they attempt to uncover the secrets of the eponymous Halo, a ring-shaped artificial world. The player can move around and look up, down, left, or right. The game features vehicles, ranging from armored jeeps and tanks to alien hovercraft and aircraft, many of which can be controlled by the player. The shield's charge appears as a blue bar in the corner of the game's heads-up display, and it automatically recharges if no damage is sustained for a brief period. When the shield is fully depleted, the player is highly vulnerable, and further damage reduces the hit points of a secondary health meter. When this health meter reaches zero, the character dies and the game reloads from a saved checkpoint. The game has been praised for giving each weapon a unique purpose, thus making each useful in different scenarios. For example, plasma weapons need time to cool if fired too rapidly, but cannot be reloaded and must be discarded upon depletion of their batteries, whereas conventional firearms cannot overheat, but require reloading and ammunition. Combat Forms result from humans and Covenant Elites who have succumbed to the Infection Forms, and have hideously deformed bodies as a result.