Anti-Healing is the exact opposite of Healing; is is the re-opening and creation of wounds and disease.
Anti-healing at lower levels can re-open small injuries. At higher levels, anti-healing can re-open larger injuries or even create entirely new injuries. In rare cases, a talented mage may be able to re-awaken a latent disease in the victim.
The notable limitation to anti-healing is that for the first several levels, it will affect only previous injuries. That being said, it is still a devastating battle-magic, particularly when used against experienced warriors.