After surviving an infection during the plague of athens (in the peloponnesian war), thucydides noted that he no longer feared the disease, "for the same man was never attacked twice." unknowingly, he was referring to

Answer is: the presence of memory cells, that are protective in case of reexposure to the disease.
Memory cells recognize bacteria, viruses that was previously in human body.
Memory cells are important because immune system has faster and stronger immune response than the first time pathogen enter into the human body.