There is a very delicate balance between how easily a virus spreads and how well it kills. The best spreading virus has you running around joyfully coughing in

There is a very delicate balance between how easily a virus spreads and how well it kills. The best spreading virus has you running around joyfully coughing in everyone's face (like common cold). The best killers die out, because you can't really infect anyone when you're dead (like Ebola). Maybe a virus can be truly airborne (not in aerosols, which do hit the floor or dry out), have a very delayed onset, such that if infects billions and then kills... But we haven't seen this in nature so I'm not really expecting us human to create something so advanced and complex in its behavior.