No one is immune to anything in life regardless of your faith or trusr. Asking why God allows? Nowhere in the bible does it say following God will leave to a stress free beautiful Harmonic life where everything goes your way and your immune. The bible is littered with stories, Joseph, job, martyrs etc.
Psalm 1:1-6 ESV
Blessed is the man who walks not in the counsel of the wicked, nor stands in the way of sinners, nor sits in the seat of scoffers; but his delight is in the law of the Lord, and on his law he meditates day and night. He is like a tree planted by streams of water that yields its fruit in its season, and its leaf does not wither. In all that he does, he prospers. The wicked are not so, but are like chaff that the wind drives away. Therefore the wicked will not stand in the judgment, nor sinners in the congregation of the righteous; ...
You're right though, they aren't immune
in that but Blessed is Blessed, which means endowed with divine favor and protection. A lot of Christians believe they are favored and protected, if not all.
Questions like the one in the OP should be asked more often so we can realize that none of us are immuned.
Here's the response... "Nobody knows."
I wish that was everyone's response to the question.