no, you dont know that they are all exposed, and that there is nothing you can do now..
throwing away the one plant is still the best solution IMO..
but if you want to treat the one plant (rather than throw it away) then at least isolate it from the resto of your collection..but dont assume your whole collection is infected if you have no evidence of that..
How many plants are we dealing with?
can you easily move the one somewhere else?