I had the following situation last night. A creature attacked with a Fire and Poison attack. My character has resist 5 Poison. does the resistance apply or do I need to resist 5 fire as well?
You need resistance to both keywords to properly resist the attack.
Against Combined Damage Types: Your resistance is ineffective against combined damage types unless you have resistance to each of the damage types, and then only the weakest of the resistances applies. For example, if you have resist 10 lightning and resist 5 thunder and an attack deals 15 lightning and thunder damage to you, you take 10 damage, because the resistance to the combined damage types is limited by the lesser of the two resistances.
The attack is Fire and Poison, as such it isn't resisted unless all the keywords are resisted.
From the latest PhB errata:
Resistance or immunity to one keyword of a power does not protect a target from the power’s other effects. Also, resistance doesn’t reduce damage unless the target has resistance to each type of damage from the attack, and then only the weakest of the resistances applies. For example, a character who has resist 10 lightning and resist 5 thunder who takes 15 lightning and thunder damage takes 10 damage because the resistance value to the combined damage types is limited by the lesser of the two resistances.