Poison and disease cards are described elsewhere (see Disease and Poison in the glossary of the rulebook [p. 43, 50]). A poison card is a type of evil enhancement card that weakens a Hero over a number of turns. A disease card works the same as a poison card, unless specified otherwise.
A poison or disease is a special ability that adds an ability to the card it targets. The gained ability is permanent, ongoing, and negative (from the perspective of the card targetted).