Why is virus called a biological puzzle?

Because when they float in air they are non living but when they come in contact with a organism they started reproducing.

bcos we can not define virus as living or non-living so it is till now like biological puzzle.

because they lie on the borderline of living and non living organism. they can reproduce only in  the host organism. when they are not in contact with a host organism they are non living . so there is a mystery about these viruses

because it is still not known to be living or non-living.

it is living and shows metabolism when inside a living organism and shows non-living characteristics when outside a living organism.

hope it helps@!!! 

Viruses are considered as biological puzzles because there is always a misconception about considering them as living or non living