The MMM Ball rush is probably one of the most annoying things to deal with, because you can't send armored units after it, because the marauders will kill it.
You can't mass it with zerglings, because if the Marauders have concussive shells, they can just kite you while the medivac units heal the little damage that you do actually pull off.
Well, we are going to tell you exactly what you have to do in order to counter the Terran MMM (marine, marauder, medivac) ball, and how you can actually use their build order to your tactical advantage! Unit Mixes that Are Strong Against This Mass Zergling - Hydralisk Mix This is great because you'll have the ranged abilities of the hydralisk, which can be quite powerful and fast acting and you will also have the speed rush abilities of the zerglings.
This combo does extremely great as well if your opponent is on your creep, because you will get speed and power boosts.
Baneling - Baneling or as I call them, Lings, are pretty great against starcraft II MMM because of their ability to bust up swarms and cause tons of hp damage in structures around them.
Roaches are great because they give you the ability to control specific units, and they are very cost effective.
This would be an extremely good counter and can be used to your advantage if used correctly.
Also Roaches are an incredible meat shield as well, so they can actually absorb a lot of the bullets and gun fire they receive and help hydralisks move into range and to do some serious damage.
Just make sure that you try your best to micro correctly!