The entire crew
The entire four hundred person crew of the Intrepid, all Vulcans, were killed by a giant amoeba in 2268. Spock was able to sense their death throes from afar and knew that none of them had understood what was killing them or would have been able to if they knew what it was.
Later, when Spock suggested that the crew had probably taken all the measures the Enterprise now was taking, Captain James T. Kirk countered that they might not have done every one of them, given the illogical nature of their situation. Spock then reiterated that they didn't know what was killing them, and added that their logic wouldn't have permitted them to even consider that they were being killed in the first place. (TOS: "The Immunity Syndrome")