If a creature planeswalker is dealt damage by a creature with infect, what happens?
Loyalty counters are different and separate from the -1/-1 counters produced by infect. -1/-1 counters affect power and toughness only. If an infected Planeswalker reverts to a non-creature, the -1/-1 counters will have no effect. They will remain until otherwise removed, or until the permanent changes zones.
Non-creature Planeswalker vs Infect Damage

Lose loyalty equal to the infect damage per rule 119.3c.
Do not gain infect counters. Rule 119.3d only applies to creatures.

Creature Planeswalker vs Infect Damage

Lose loyalty equal to the infect damage per rule 119.3c.
Gain -1/-1 counters equal to the infect damage per rule 119.3d.

Example
Gideon, Champion of Justice has five loyalty counters and five -1/-1 counters. If he resolves his zero ability to become a creature, Gideon will die the next time state-based actions are checked.
Planeswalkers are not subject to the same state-based action where a player with ten poison counters automatically loses the game. Planeswalkers cannot gain poison counters.