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.

On 4th august, 1914 britian declared war on germany .The ww1 ended ww2 had begun. less than six years later ensuring peace that followed was labelled 'the peace to end peace ' as if to make this prophetic statement came true , hitlers armies invaded poland twenty years later , on 1st September 1939. two days later, Britain and france upheld their guarantee to poland and declared war on germany for the second time in 25years.japanese aggression in the far east and pacific ww2 into a global conflict by 12 December 1941.when ww2 ended ,it mourned the death of about 70 million victims and witnessed the introduction of the most feared weapon ever invented in the history of mankind - the nuclear bomb.