Final answer:
The Plague of Athens killed one-third of the Athenians inside the city walls, including Pericles, during the Peloponnesian War. It was caused by a pathogenic bacterium that quickly spread in the overcrowded city.
Step-by-step explanation:
The disaster that killed one-third of the Athenians inside the city walls, including Pericles, was the Plague of Athens. In 430 B.C., the plague struck Athens during the ongoing Peloponnesian War. It is believed to have been caused by a pathogenic bacterium, Salmonella enterica serovar Typhi, that causes typhoid fever. The disease spread rapidly in the overcrowded city and resulted in a significant loss of life, leading to the weakening of Athens's dominance and power.