And one more thing. If you Zergs find marines to be oh so OP and everything, why not just Neural Parasite them and use them against your opponent? I mean, they're so overpowered right? A little of your Infestors's energy doesn't matter if you get to use Marines.
All jokes aside, Terran is all about being aggresive and harrassing. Zerg is really harass prone, so you should drop against Zerg a lot more often than the other 2 races. 1 Medivac is all you need for harassment and you should NOT send any more than 1 medivac at a time on the same place if all you want to do is harass. IMO you are harassing effectively if you get to your opponent's base, kill plenty of workers/a vital tech structure/Queens/Hatches and Overlords AND manage to get away if things get too bad so that you still have those marines to use either for harassing again or so that you can add them back to your main army. That said, if you get to focus your marines on a hatchery until it has really little HP left, but the Zerg's army is coming to where your dropped marines are at, go ahead and kill the hatch first if you can, then leave and hide your marines somewhere else.
One more thing. Never ever ever EVER give Zerg map control. Scan and kill creep tumors by sending out a few marines (3-5 are enough) when you are about to move out.
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