Virus

The plot of the game is reminiscent of the arcade game Defender, in that the player, piloting a lone craft with limited firepower, must defend a finite landscape against ever increasing waves of enemy craft. In Zarch, the landscape is being invaded by aliens who are spreading a virus across the landscape. The seeder vessels are slow-moving, predictable, and easily destroyed, but as the game progresses they are supported by increasing numbers of flying support craft, which do not scatter virus but instead attack the player.

Source: TheGamesDB