During the 14th century there was a plague in Italy, and St. Roch came upon one of the towns most affected by it.
According to the Catholic Encyclopedia, he “devoted himself to the plague-stricken, curing them with the sign of the cross. He next visited Cesena and other neighboring cities and then Rome. Everywhere the terrible scourge disappeared before his miraculous power.”
He eventually contracted the plague himself, but after retreating to the forest, he was also cured from the disease.
After his death, St. Roch’s intercession was invoked when a plague struck Germany in the 15th century.
In 1414, during the Council of Constance, the plague having broken out in that city, the Fathers of the Council ordered public prayers and processions in honor of the saint, and immediately the plague ceased.
Time and time again he was invoked during various medieval plagues and is why he is labeled a patron against plagues.
During a deadly plague, Pope Gregory had this consoling vision of St. Michael the Archangel