What? How? How discord got infected?

That's impossible.

12034855
Wouldn't exactly say impossible.

Possible Spoiler prominent message:

Discord has definitely gotten sick before (although it could very well been an easy ticket to annoy Twilight when Cadence visited her). This infection is most related to a prion disease, or specifically Chronic Wasting Disease (CWD) commonly found in deer. It could also be related to Bovine Spongiform Encephalopathy (BSE) or better known as Mad Cow Disease.. which is also from a prion. Which you guessed it, affects animals/mammals.

This is also an ALTERNATE UNIVERSE, which usually means the universe is not connected to canon, and may or may not have themes of characters being either out of character, or are in a situation in which the canon would likely not be prominent or existent at all. I understand Discord being infected may come as a shock, but that is one of the prominent themes of the story. If you don't agree with it, that's totally fine!

12035037
Okay, i get that he can get infected but now my real question is, how the hell he got infected? I don't think that the walkers can get to touch him or even scratch him.

12035579
Spoiler prominent themes for possible future chapters.

The walkers aren’t like your typical Walking Dead zombies. They’re more of a mixture between the infected in All of Us are Dead and World War Z. So walking up to Discord and biting him would likely not work at all. However, charging at him full-force definitely could.

We’ve established the mirror stopped working in both Equestira and the EG universe. Chances of magic being limited or fully erased is likely, but that will also bring up the question of how Discord got to EG in the first place.

My answer: Discord’s magic is a bit more complex than the average unicorn (I consider Twilight having magic a bit more complex than unicorns, but still not borderline alicorn till her complete transformation). Discord’s magic is still working, even while infected, as the mutation is a cause of Discord infecting other survivors. Think of it as the T-800 virus from Zombieland, but the host of the mutation is from a much more powerful subject.

In the future I may tackle a chapter/sequel/or extended cut of Discord’s POV when he first arrived to the now apocalyptic EG universe... but I am prioritizing Sunset's story beforehand. I hope this answered some of your questions.