This game is super fun and feels like a true arcade game. I like the idea of the virus multiplying and raging when there is only a few of them left. Everything felt really good, I also liked the weapon swapping mechanic, this game overall feels very polished and I really enjoyed it.
If you wanted to improve, I suggest having the score do something like unlocking the other weapons for the rotation, that would give the player a sense of forward progressing and a reason other then getting a high score to keep playing.
Overall super fun game!