Short answers, pulled from the story.
The Black Death arrived in Italy during October 1347 when twelve Genoese ships docked in Messina, Sicily. The plague migrated from there toward Catania becoming the second major center on the island and spread to Northern cities like Genoa and Venice throughout spring months of 1348.
Giovanni Boccaccio wrote about Florence when the autumn of 1347 brought rumors of a significant epidemic. He described how streets were cleaned and travelers with symptoms denied entry into the city while the plague lasted until July despite these efforts.
Sicily became the first point of contact with twelve ships arriving in Messina during October 1347. The harbor held vessels fleeing the siege of Kaffa in Crimea where infected corpses had been catapulted over city walls by Mongol forces.
Only eight survivors remained in Siena when the plague finally departed after Agnolo di Tura chronicled the abandonment of loved ones whose bodies were dumped into holes without tears. People contracted illness through minimal contact even by touching clothes or objects handled by the dead.
Italian city-states developed early public health regulations following the initial wave of infections including ship quarantine measures that appeared in port cities. These regulations banned travelers from infected zones from entering urban centers entirely and formed foundations for modern quarantine law regulation between 1347 and 1353.