After level 7, you should be burst healing for 500 (scaled by level) every 5 seconds. Mid-late game, that gets a 50% chunk when finishing your level 4 quest. Holy ****.

The combo: Q, wait until it spreads to your allies, W -> D. If more than one enemy is near, you must hit D before the infection spreads (every 0.75 seconds) or your D will go on full cooldown.

You can actually aim your W at more than one enemy in a line if you have the reaction time to hit D before the second enemy is hit. This helps stack your level 1 quicker.

Your E is only used in-between your basic combo. With your lvl 13 Lingering Spines, you can toggle it quickly and get back to your micro. Ideally you want to save it for critical moments (canceling an ult or an escape ability) but if you know a fight won't be soon it's got more DPS against 2+ targets than Stukov's AA, so use it on mercs and structures if you can hit more than one at a time. Once you're down to one target, switch to AA.

Level 1: Fetid Touch is important to make your combo ready as soon as possible, but it's not strictly necessary. Growing Infestation and Poppin' Pustules are both good for clumped enemy teams with many bodies, e.g. Samuro, Rexxar, TLV.

Level 4: Vigorous Uptake all levels where you can be with 2-3 other allies frequently. Basically any map that isn't BHB, Warhead Junction, or sometimes DK. On those maps, Biotic Armor synergizes well with lvl 16 Pox Populi.

Level 7: Necessary or pick a different build.

Level 10: Until the cooldowns are normalized, Massive Shove.

Level 13: Anything except Reactive Ballistospores, which are anti-synergy with Targeted Excision. Lingering Spines is nice.

Level 16: Pox Populi unless your enemy has lots of stuns (Superstrain) or lots of AA (Eye Infection)

Level 20: Currently, the two ult upgrades are almost strictly nerfs.