What brought the plague to Catania?

People who emigrated from Messina brought the plague to Catania.

Messina was one of the first locations on Sicily to be riddled with plague. Many people died, and many thought they weren't contaminated with the plague, so they decided to emigrate from this disease-riddled city. That's why they went to Catania, the city which the plague still hasn't reached. However, it turned out that some of these emigrants indeed had the plague, and so they contaminated Catania as well as Messina.