This was a cool idea! I don't think I've seen this sort of collective health thing before, but it's really interesting. I liked the hand drawn visuals, I think that was a really nice style to go with, especially the main menu, that was done well. The game was pretty clear and easy to learn how it worked.
Some little things to note are that sometimes the enemies could spawn on top of you, which was a little bit annoying. Also, next time PLEASE make sure the sounds are at a reasonable level, that scream made me hit my knee on my desk.
With a little more polishing, I think that this would be really cool if expanded on! Good job!