This game Is pretty cool however there is a few things that you could add to turn this good game into a great one!
1. To fix the slippery movement I would recommend using .GetAxisRaw() instead of .GetAxis() in order to cancel your characters momentum when it stops moving.
2. Unity has a really amazing particle system and it can be used to cover up the frame where you despawn the sprites which makes the collisions look awesome.
3. As Mark Brown says, you need a consequence to your main mechanic. When would you NOT want to shoot. Maybe you could have patients trying to escape the virus and shooting them looses you lives.