Melira, Sylvok Outcast has three static abilities that do similar things, in that they all ‘shut down’ infect. The first ability prevents you from receiving poison counters. The second prevents creatures you control from receiving -1/-1 counters. The last ability removes infect from all creatures that your opponent’s control.

While the first two abilities prevent counters from being placed on a player or creature, these two abilities do not remove any poison counters that a player had or -1/-1 counters that were on a creature. If when Melira came into play you had 7 poison counters and a Sylvok Replica you controlled had two -1/-1 counters, they will remain. All that Melira does is prevent you and your creatures from receiving any more.