Marvel Zombies/WMG

Ms Marvel is actually patient zero, not Sentry.
Moonstone/Ms Marvel is having sex with every single sapient/near-sapient entity in Marvel. The amalgamation of every STD in the known Marvel Universe will be brought together onto one single body and give way to something far worse. Alas, poor Sentry picked the absolute wrong spot to appear on "Ms Marvel's to do" list. He catches the virus and meanwhile in space he turns. The rest has been foretold.
 * Jossed.

Patient Zero is not actually Sentry, but Superman
DC Zombies. Nuff said.
 * Naaaaaaaaaaaaah.

Marvel Zombies is the result of Superboy Prime's punching time
The reverberations from his temporal Wangst were so far reaching that they managed to merge fragments of the old EC universe with that of the alternate earth from which the zombified Sentry originated.

Zombie Deadpool isn't really a zombie
Surprised I'm the first one to push this. Deadpool is well known for not playing with a full deck, so, while strolling in the park one day Wadie notices that other super-guys are starting to go all Night of the living dead. Not wanting to be left out, grabs the closest person to him, who turns out to have been infected. While chomping down on his little snack, a piece of flesh is caught in his teeth, and he is covered in blood/gore. And there you go, a clinically insane man is convinced that he is a zombie. But wait, what about the whole zombie mutilation thing? It is possible that his healing factor is not automatic, so if he thinks he's a zombie, there is no need to heal. Add pretty bow here.
 * I have no trouble believing this one actually.
 * Deadpool has lived through decapitation before, albeit with slight Drain Bamage.

Frank West will stop the zombies.
He's covered Civil Wars you know.
 * Needs to be canon for Marvel vs. Capcom 3.
 * It could only be cooler if he teamed up with Chuck Greene to take them down.
 * Well, damn. Look at his ending in Ultimate MVC 3. Reed Richards sends him in for exactly that!

The zombified Sentry came from the universe of Marvel VS Capcom 3

 * Here's the thought- we know Sentry punched his way into the Marvel Zombies universe from another one, already being infected. This infection is actually a heavily mutated form of the Uroborous Virus perfected by Albert Wesker by experimenting on the Marvel superheroes of his universe (as seen in his ending from Marvel vs. Capcom 3). Sentry escapes and punches his way into this reality, leading to all that follows.