In all but the most excruciatingly formal language, people would usually say “Who do you fear for?”
Just about the only times you’ll hear or see the word whom would be:
grammar books
tests
in extremely formal English
directly after a preposition (but because we like to hang our prepositions on the ends of sentences, this doesn’t happen too often in everyday language)
One more question.
If I need to be more specific, can I ask a question in that way (for example):
Whose <health,…> do you fear the most?
(Let’s suppose that we both – the speaker and the listener – understand clearly of whom we are talking about (those people I mean).