can someone explain to me how they get infected again?

transmitted by blood

so like how do the majors not get infected when they get adam blood on them all the time?

plot armor, also they do. In the beginning Major kwak mentioned that he used to have over 100 brothers and sisters in arms but all of them are gone except for 4 including himself, Major lee, Major yang and Major yoo (Major yoo doesnt go onto the field cause he a bitch) Major Yang, Major Kwak and Major Lee are all S class soldiers part of the "immortal regiment" the word "immortal" coming from them surviving for a long time.
the rest is plot armor (￣∇￣")

thank you twin