The duchy of Milan was ruled by the Visconti during the black death. In 1374 Bernabo Visconti gave some regulations regarding what to do in case of plague symptoms. Anybody with symptoms of the plague was forced to leave the city, and parish priest had to notify of any sick people, and their items became property of the lord. I'm not sure that the regulations were effective per se. The isolation of the duchy due to war is cited among the other possible explanations.

In any case, Milan was not immune from the plague. Gian Galeazzo died of bubonic plague in 1402, and Caterina probably died for the plague too. Furthermore, the plague in 1630 afflicted the city regardless of any regulations in place.

Sources: Beltrami, Luca. Il Castello di Milano:... sotto il dominio dei Visconti e degli Sforza, 1368-1535. U. Hoepli, 1894

Chronicon regiense, rerum italicarum scriptores XVIII, Milan, 1731, col. 82

I'd like ask a question of my own, which I always found very strange and funny among myself and my friends, but how come the Polish area was so uneffected by the Black Death? It seems like such a strange area for the disease to go around if you don't know why.