xxx.1. decease represents four abilitys - two triggered and two static abilitys.
xxx.1.a The first one is a static ability that reads "This creature comes into play with a pest counter on it."
xxx.1.b The second one is a static ability that reads "If this is a non-artifact, non-zombie creature, it gets -1/-1".
xxx.1.c The third one is a triggered ability that reads "At the beginning of your upkeep put a pest counter on target non-infected creature, that is not targeted by another pest ability, you control.
xxx.1.d The fourth ability is a triggered ability that reads "When this creature deals combat damage to a non-infected creature, put a pest counter on that creature."
xxx.2. A creature with pestilence is called "infected creature".
xxx.3. If a creature with pestilence loses pestilence, all pest counters are removed from it and vice versa.
xxx.4. Multiple instances of pestilence are redudant.