Rugal's tips are great. Just to emphasize them a little more, I believe Airborne is the primary engine of the entire Infector deck. The fact that it can transform every single monster attack into something useful regardless of how far away the target is, or how much armor they have, or how poor your luck is on dice rolls cannot be overstated. It is kind of broken. Even when you aren't building around it, it makes rolling the dreaded X never a waste.
If Airborne is the engine, Outbreak is the big payoff. On any map where you can set up a Ranged monster group to safely spend a few turns within line of sight on the heroes, but well out of reach, it is trivial to get a large number of Infector tokens on them. This is even easier on quests with more than one encounter, as Infection tokens carry over. If you focus on a hero with low Might, and follow up with an Onslaught empowered by a card like Befuddle to ensure the skill test fails, you can basically one-shot any Hero you want, when you want, on THEIR turn, which is the worst possible time for them, because they lose their turn completely even if they're revived by another hero right after. This is a very nice answer to something like an Act 1 otherwise unkillable Mage like Ravaella wearing Runeplate.
The Magus deck and other deck manipulation cards like Upgrade and Refresh will help you ensure you always have your Airborne out as soon as possible.
Dark Host and the other Infector cards are the icing on the cake, but focus on Airborne and Outbreak, imo.