The red swollen area on his back was confirmed to be a carbuncle caused by staphylococcus infection.

carbuncle 来自拉丁语 carbunculus（小煤块），由 carbo（煤）和后缀 -unculus（表示"小"）组成。在医学上指"痈"，即由葡萄球菌感染引起的多个相邻毛囊的化脓性炎症，表现为红肿疼痛的肿块。