Melira doesn't remove any poison counters you already have or any -1/-1 counters already on creatures you control.

It's possible for a creature an opponent controls to gain infect after Melira enters. For example, your opponent could cast Glistening Oil on a creature, and then Melira's ability and Glistening Oil's ability would be applied in timestamp order, with the most recent effect winning. The damage dealt to you by such a creature won't result in you losing life (because the creature has infect) or in you getting poison counters (because Melira stops them). Similarly, damage dealt to a creature you control by such a creature won't result in damage being marked on the creature or in -1/-1 counters. However, abilities that check whether damage was dealt (such as lifelink or Whispering Specter's triggered ability) will still see that damage.

Damage dealt to planeswalkers you control causes loyalty counters to be removed from them, regardless of whether Melira is on the battlefield.

If a creature with infect enters under an opponent's control after Melira enters, that creature loses infect.