The Super-Soldier Serum is actually one-half of a process used to turn scrawny little Steve Rogers into the dominating power-house known as Captain America. The serum, when combined with vita rays, created a transformative effect that increased muscle tissue and density in a living subject on a permanent basis. The formula was developed by Doctor Abraham Erskine and employed by the United States Army in the early 1940s as part of top secret government program called Operation: Rebirth. Operation: Rebirth was compromised by a Nazi agent who assassinated Doctor Erskine. Steve Roger survived and the spy was terminated, but the secrets behind the formula died with Erskine.
In the ensuing years, the military continued their efforts to duplicate the Erskine process. They forcibly recruited more than three-hundred black soldiers and used them as test subjects for an updated version of the serum. Very few of whom survived. One who did survive was Isaiah Bradley who became the "black" Captain America for a time. Isaiah's grandson, Eli Bradley, would eventually become known as the Patriot.
Years after World War II, an American scientist named Ted Sallis attempted to recreate the super-soldier serum on behalf of the U.S. government. He worked in a small laboratory in the Florida Everglades with his assistant and lover, Ellen Brandt. Sallis' attempts to duplicate the formula back-fired when he discovered that his backers were actually part of Advanced Idea Mechanics. Desperate to keep the SO-2 serum formula out of A.I.M.'s hands, he injected himself with it, then drove his car into a swamp. Contact with the swamp water had an unpredictable effect, transforming Ted Sallis into the Man-Thing.