How is it that Col. Nick Fury was able to fight the Nazis during WWII, the Vietcong during the Vietnam War, become the leader of S.H.I.E.L.D. and still remain as strong and vital as the day he stepped out to meet his "Howling Commandos"?
Nick Fury has been able to thrive for all these years thanks to Professor Sternberg and his Infinity Formula.
During a routine mission through the French countryside during the height of World War II (as told in Marvel Spotlight #31), Fury stumbled into a mine field and was critically injured. He would have died right there in France if a group of partisans hadn't brought him to Sternberg for treatment. While being treated, Sternberg used Fury as a guinea pig for his experimental serum, which once it was used, must be taken yearly to sustain near immortality.
Or so the story goes... and now you know. Of course Nick fury was white in the comics and is now an African American in the movies. Nick Fury that we see in the movies is from the ultimate series and not the regular Marvel line.