WARNING: The following contains spoilers for What If...? Season 1, Episode 5, "What If... Zombies!?," streaming now on Disney+.
What If…? has established the twist ending as standard for its various forays into the Multiverse, taking a page out of The Twilight Zone’s books. The intent seems to be to remind viewers that change is constant in its bizarre variations on the Marvel Cinematic Universe. Season 1, Episode 5, “What If… Zombies!?” came up with a particularly chilling final image: a zombified Thanos leading an army of the undead against Wakanda, just as hope for a cure flies towards it.
It makes for a strong visual punch to end the series’ darkest episode, but it also requires a little unpacking. “Zombies!?” takes place during a very tight timeline, and as unsettling as it is, certain things need to happen at a certain time in order for Thanos to be a zombie at that point. That helps frame the logistics, but turning arguably the most powerful being in the MCU into a mindless flesh eater still requires a little more explanation.
The virus in “Zombies!?” moves extremely quickly, but more or less originated on Earth and remains confined there when Thanos arrives. (That point in time roughly matches the events of Avengers: Infinity War, with a few key differences.) Even if the virus had spread beyond Earth, however, Thanos would presumably still need to be among the living to gain the Soul Stone, since a zombie is incapable of making the required moral choice. That means that – with the Power, Reality and Space Stones already in his possession – he needs only two more to make the Snap. However, in this reality, Doctor Strange was never abducted by Ebony Maw and the Time Stone thus remained on Earth.
It’s therefore safe to assume that Thanos arrived in New York in search of the Time Stone, only to find it lying on the street along with the skeletal remains of Doctor Strange. The Wasp presumably had no idea of the Eye of Agamotto’s value when she killed zombie Strange, leaving it for Thanos to pick up and claim unopposed. Presumably, he suffered the wound that turned him into a zombie shortly after that.
That's when the realities of his invulnerability start to cause awkward questions. Thanos has already shown himself to be stronger than the Hulk, and the Hulk is immune to the virus simply because he can’t be hurt by a bite or scratch, no matter how powerful. What brought Thanos to undead-dom so quickly that he had time to rot a bit before arriving at Wakanda? The list of figures that might hurt him is short, though members of his Black Order such as Proxima Midnight might be able to, if he encountered one of them as a zombie. Wanda Maximoff might be able to as well, and indeed, she might have bitten Thanos as he attempted to take the Mind Stone from Vision, which would fit the timetable of Infinity War.
Another possibility is Hank Pym, who could conceivably shrink down small enough to slip beneath Thanos’ skin, then grow large enough to begin eating his more vulnerable internal organs. Even if they resisted damage as well, it might be enough to spread the virus, and with Thanos unaware of the danger, it could readily explain the transformation.
What happens next is open to speculation, of course, which is one of the reasons What If…? employs such dramatic turnabouts. Presumably, the shot of zombie Thanos is intended to imply the kind of universal zombification suggested in the Marvel Zombies comic series on which the episode is based, and a Snap from a zombified Thanos might very well result in such a condition for the universe. Alternately, Thor and the Guardians could destroy him properly before he reaches the last Stone – Thor would likely aim for Thanos’s head this time – and as the show delights in reminding viewers, a universe exists somewhere where each of these possibilities have happened, in countless different ways. That expression of possibility leads to images like zombie Thanos, which in this case requires more explanation than normal. What If…? settled for inferring it rather than stating it, leaving fans to perform the required mental gymnastics on their own.
To see the Marvel Cinematic Universe overrun with zombies, the newest episode of What If...? is streaming now on Disney+.