Force 3 was a setting available on 23rd century phasers. It was powerful enough to destroy most living organisms. A notable exception was the neural parasites that invaded Deneva in 2267. Because each parasite was part of a huge organism, and drew strength from that organism, it could largely resist even Force 3. Parasites exposed for five to ten seconds reacted as if mildly stunned; they fell from the surface to which they were attached, and did not react to external stimuli for about one minute. (TOS: "Operation -- Annihilate!")