If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
In this game you are a vaccine. You are moving in some micro space and meet many bacteriums on your way. Use your weapon to shrink these bacteriums. Once they are small enough, you can eat them and get more power.
Arrow Keys / W, A, S, D - Move.
D / H - Fire..
S / J - Eat.
A / K - Fire Secondary Weapon.