"Do you really wish to know?" — Spoilers from the books and/or adaptations to follow!
Gert Borel was a peasant that hauled plague victims' bodies as he was confident he was immune to it considering all his life he never got sick. However, later on he was found wandering the battlefields of southern Velen (near a burned-out settlement east of Reardon Manor), begging for death, having come down with the plague himself. Geralt can choose to end his suffering and burn his body.