WordNet sense 1 (having the ability to perceive or understand; keen in discernment):

perceptive

WordNet sense 2 (be difficult to detect or grasp by the mind):

elusive

WordNet sense 3 (imperceptible to the senses or the mind):

impalpable

WordNet sense 4 (working or spreading in a hidden and usually injurious way):

insidious, pernicious

WordNet sense 5 (causing or capable of causing harm):

harmful

WordNet sense 6 (exquisitely fine and subtle and pleasing; susceptible to injury):

delicate