Thank you for your review! We're glad to see that you're enjoying the game! :D
As for the crawlers, they're meant to be difficult (they're usually what get me). The good news is that they usually travel in swarms. Because of this, they can be easily taken out by placing a sentry turret in the middle of their swarm (It's dangerous, but highly effective).
After the end of each round, every citizen in the city (including you) will be vaccinated. This way you don't have to waste your antidotes. I would suggest saving your antidotes for yourself or the last remaining citizens. We will be sure to clarify this confusion in our next update! :)