How did they get away with it?
Based on the screenshots ( http://www.mobygames.com/game/virus_/screenshots ), it doesn't look like the original Virus was trademarked (no TM or ® on the name).
Japanese companies (like Space Invaders' Taito) apparently haven't been big on enforcing their trademarks or copyrights in the past: http://www.gamasutra.com/features/20061024/andersen_01.shtml