LÖVE

You start as a spirit that can infect zombies to control them to kill other zombies. As you kill other zombies you gain bloodlust stacks (yellow bars) that increase your damage and movement speed. On +10 killstreak (S) you gain lifesteal. Each wave zombies become faster and have more health.
You can eject from a body to infect another zombie, but you will loose killstreak and bloodlust stacks and it will consume one infection charge. You will eject if your body dies. Infection charges increase once per 10 kills.
If you eject from a body while having 0 infections - game is over.
Spirit is invulnerable, but has limited lifetime, and if it dies - game is over.

WASD - move
Left mouse - infect (as spirit when touching a body) / attack (as zombie)
Right mouse - eject from a body (kills body instantly)

This is a minimalistic zombie wave beat'em up that has been done in ~6 hours as my GTD management attempt. There will be no further development of this, because i claim it as "finished" (for myself) so i won't stuck in permanent improving cycle, but you are free to dissect is as you like.

Sprites are mine and also taken from OpenGameArt (credits in txt file), Modeseven font is under openfont license.
Gameplay video

made for love 0.10.1