I would think that they had begun to build up resistances to the virus's and conditions they were exposed to.
The correct answer is Bernard of Clairvaux
He was a French abbot born in the final decade of the 11th century and lived and worked during the 12th century. He was known for establishing the reforms of the Benedictine monasticism as well as for forming the Cisterian order. He was a very important person in the era because of his participation in debates on the schism and similar things.
