An anal fissure usually heals within 6 weeks (known as acute anal fissure). But if it persists for longer than 6 weeks, it is known as a chronic anal fissure. An acute illness is typically over quite quickly. A chronic illness is one that lasts a long time, sometimes for the rest of the affected person's life. When describing an illness the words 'acute' and 'chronic' refer to how long a person has had it and not to how serious the condition is.
Attribution: BUPA