Can you proliferate poison counters on an opponent who has zero poison counters?

Proliferate means you choose any number of players or permanents that already have at least one counter on them, then put one additional counter of each kind those chosen players or permanents already have on them (CR 701.27a). The key word is 'already have' — if a player has zero poison counters, they are not a legal target for the poison-counter part of proliferate.

So if your opponent currently sits at zero poison counters, you cannot use proliferate to give them their first poison counter. Proliferate copies existing counters; it does not grant new types of counters to players or permanents that don't have them yet.

If, however, your opponent already has at least one poison counter — say, one poison counter — you may choose them when you proliferate, and they will receive one additional poison counter, bringing them to two.

Concrete example: You cast Prologue to Phyresis, which deals 1 poison counter to each opponent, then lets you proliferate. If an opponent just received that first poison counter from Prologue's first effect, they now have 1 poison counter, so you can choose them during the proliferate step to bring them to 2. But if an opponent somehow avoided the first effect and still has 0 poison counters, you cannot choose them to add a poison counter via proliferate.

This rule applies equally to all counter types: proliferate can never introduce a counter type that a player or permanent does not already possess (CR 701.27b).