Abscesses are usually caused by bacterial infection. They are most often seen on skins. The symptoms are likely to recur after healing.脓疮通常是因为细菌感染引起，最常见于皮肤上，愈后很容易复发。Not all chemicals normally present in living organisms are harmless.并不是所有常见于生物体中的化学物质都无害。This liver condition is common in heavy drinkers.这种肝病常见于酗酒者。The disease is common in children.该病常见于儿童。