In Captain America: The First Avenger, HYDRA head Johann Schmidt tested a version of Abraham Erskine’s Super Soldier Serum on himself.
However, the effects saw the Nazi become terribly disfigured and gain the name Red Skull.
Skinny little Steve Rogers was injected with the serum too, but the results were very successful, as he became the super soldier of the Avengers we know today.
But now a popular new fan theory has argued why the Red Skull was deformed by the serum but Captain America wasn’t.
Reddit user rafael-a wrote: “Bacteria is the answer.
“In the scene of the transformation of skinny Steve Rogers into big buff Captain America there is a small joke where Erskine injects him [with] penicillin beforehand.
“The penicillin shot was much more important than it seems, because [the] human body is full of bacteria, and the Super Serum would enhance microbes as well.
“Therefore they should be killed beforehand.”
As for Captain America, he ended up going back in time and having a long life with Peggy Carter.
Steve Rogers is now well over 100-years-old in the MCU but could he return one day?
Well back in June, Chris Evans revealed he would return to the franchise on one condition.
Speaking on The Graham Norton Show, the 39-year-old said: “It’s not a hard no, but it’s not an eager yes either.”