[Starbelly Geek]

Sick head means diseased.
Diseased is an affliction, so you'll also have a little heart, and the little heart will tell you about what it means to be diseased.

Diseased people lose a hit point per month (unless they are regenerating, in which case they lose a hit point every Late Winter turn) and have a chance every month of getting another affliction.

Other afflictions cause other side effects that generally don't involve the continual loss of hit points. Only "diseased" has an extra obvious indicator, although some are apparent from the stats (crippled = 2 AP, battle fright has lowered morale, blind has lots of zeroes) or description (lost limb means you only get one weapon and no shield).