Because for your body to fight a virus, it needs to have a good working immune system. Some people lack that for various reasons. A vaccine helps in this case b

Because for your body to fight a virus, it needs to have a good working immune system. Some people lack that for various reasons. A vaccine helps in this case but it's not 100%. Being "fit" certainly helps your immune system. But as we see here, it doesn't make you invincible.