Some people have better immune systems than others. Unless you know that you have an autoimmune disease, it could actually just be that you have a good immune system.
The attitude that “you just have to have faith and bad things won’t happen to you” (implying that people who get sick or suffer lack faith) is more Protestant than Catholic. Along with having overtones of the “health and wealth gospel” which is contrary to the true Gospel, it suggests that faith is something we do, rather than an infused virtue - a gift from God - as the Church teaches.
St. Damien of Molokai volunteered to go work in a leper colony, not because he had faith that he would not get leprosy, but because it needed to be done and he was willing to risk himself. He did contract leprosy and died of it, and the Church canonized him, so obviously it does not consider him to have lacked faith.
Miracles are possible but they are a gift from God, not something we earned. Thank God for giving you such good health and stop judging others who are trying in prudence to protect themselves.