The character of Steve Rogers gets contracted with polio in childhood a deadly disease and because of which his body doesn’t develop normally. Somehow he survives even after being badly hit with polio. Even after knowing the fact that his body doesn’t have that much of physical strength he determines himself to serve in the U.S Army.

Arm forces require people who are good with their muscles and strength as they have to be on the battlefield so he gets rejected so many times there too. But in between those rejections and inferiority, he develops indomitable spirit, a strength of will and tries over and over for serving in U.S army and fight in World War II desperately.

After hit by endless rejection finally, he founds a shelter in a science project Rebirth which was an experimental program which according to the scientist was capable of building up his weak body into a normal one.

Scientists actually created a serum which was capable of converting a normal person into a super soldier. The project rebirth was not a successful one in its previous attempts but that project apparently works on Steve Rogers. 

He becomes a symbol of patriotism after becoming a super soldier and serves in the U.S army in World War II in his style and being an important soldier he gets renowned as Captain America.