When an adjective is used as a predicative (i.e. as the “object” of a verb like sein or werden), it is always in its base form. No need for conjugation.
so, " are you sick?" will be "bist du krank?".........is it correct ?
Or, of course, "Seid ihr krank?" or "Sind Sie krank?" -- depending on how many people you are talking to and how well you know them.