If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
Shoot zombie fishes they are approaching towards you. Kill as many as you can, remember beware of them, stay away from infected fishes otherwise you will be quarantined. Collect ducks to get new weapon or ammo and don’t forget to take antidote if you are infected.
Arrow Keys - Move.
S - Fire.
A - Auto Fire.