While walking along the road near Gvozdno, some infected started attacking me. When I tried attacking them, they were invincible not taking any damage or even being stunned when being attacked. After running away and trying to shoot them with a slug shotgun BK-133, it still had no effect after clearly hitting them and I was knocked out soon after and killed. To further add to the report, the infected were moving faster then usual, and seemed to bounce up and down as if clipping into the ground