A character can be poisoned in a number of ways. The "easiest" way is to set off a Poison Needle trap on a chest. There are monsters in deeper levels of the dungeon that, when they hit a PC, have a chance to poison them. Chests in deeper levels can also have Gas Bomb traps which have a good chance of poisoning every member of the party if they are set off.
When a character is poisoned, his hit points diminish gradually until he is cured or leaves the maze. Inside the maze, the poison's effect does NOT wear off after some time like in other RPGs. However, while poisoned, the character can still fight, cast spells, identify items, be healed, and everything else he could otherwise do while OK.