Rather than explain the rules in great detail, I'll simply provide an example. There are two venomous snakes attacking a wizard. They have the following venom:
Black adder venom: injury DC 11 1/rd. for 6 rds. 1d2 Con 1 save cures
- The snakes attack. One hits. The wizard rolls his save (DC 11) and passes. He is not poisoned.
- The wizard casts magic missile!
- The snakes attack. One hits. The wizard rolls his save (DC 11) and fails. He is now poisoned (6r remain).
- The wizard takes a five foot step and drinks a potion. He rolls a save (DC 11) and passes. He is cured.
- The snakes attack. One hits. The wizard fails his save (DC 11), and is poisoned (6r remain).
- The wizard casts magic missile! He fails his save (DC 11). He takes 1d2 con damage, and is still poisoned (5r remain).
- The snakes attack. Both hit. The wizard fails both saves (Both DC 11). The save DC against the poison on his turn is now 15. The duration of the poison is extended to 12 rounds total (11r remain).
- The wizard casts magic missile! He fails his save (DC 15), takes 1d2 con damage, and is still poisoned (10r remain). One snake dies.
- The other snake attacks. It hits. The wizard rolls his save (DC 11) and passes. His poison is not made worse, but he is not cured.
- The wizard casts scorching ray! crit! He fails his save (DC 15), takes 1d2 con damage, and is still poisoned (9r remain). The snake dies.
- The wizard fails 2 more saves, and dies. He should not have gone adventuring alone.
This example illustrates pretty much every case about which I've seen confusion in the past. You don't suffer the poison's effects until your turn, whereas whether you are poisoned or not and the dosage is decided on the enemy's turn. This is stated in the afflictions section, emphasis mine:
The affliction's effect does not occur until after the onset period
has elapsed and then only if further saving throws are failed.
This poison and many others have no onset, so the afflicted character must begin making saves immediately on his turn. In no way does the lack of an onset imply that ability damage is taken the instant poison is contracted. This is inconsistent with all other afflictions and essentially adds one more ability damage roll to a poison that it should not. The roll to determine whether or not you are poisoned is entirely distinct from the roll to determine the effects of the poison.
The distinction between contracting and curing a poison is important because as poisons crop up with more complicated cure conditions, sticking to this consistent set of rules will make things much less confusing. For example, a phase spider's venom requires two consecutive saves to cure. If you took ability damage immediately on failing the first save, that would imply that even if you passed you would still be poisoned and have to save again. That is incorrect. If you make your save against contracting a poison, it does not affect you, regardless of cure condition. By keeping the rolls for contracting and curing separate, you avoid this type of confusion entirely.