Lines are quoted from the official notes; buff/nerf tags are auto-classified from the numbers and can be wrong. Covers the 14 most recent Dota 2 patches PatchMeter has fetched.
▲buffVenomous GaleIncreases cast range and projectile speed by 25%. Creates 2 Plague Wards around every enemy hero hit
⇄adjustNoxious PlagueAbility reworked
✕removedNoxious PlagueNo longer has AoE effect, now affects only the host
⇄adjustNoxious PlagueNow when the plague spreads, it also carries all debuffs placed by Venomancer
✓fixNoxious PlagueDoesn't stack. Applying plague to an already plague-infected unit will deal projectile damage again, but won't affect the remaining debuff duration. Duration of carried debuffs is fixed and cannot be altered with Status Resistance or Debuff Amplification
▲buffNoxious PlagueMana Cost decreased from 200/300/400 to 200/250/300
▼nerfNoxious PlagueDuration decreased from 5s to 4s
▼nerfNoxious PlagueInitial Damage decreased from 200/300/400 to 150/200/250
⇄adjustNoxious PlagueInitial Damage is now non-lethal
⇄adjustNoxious PlagueNow spreads a second time, but all spreads after the first one deal no initial damage
▲buffNoxious PlagueSpread Radius decreased from 800 to 700
▼nerfNoxious PlagueMax HP as damage decreased from 3/4/5% to 2/3/4%
⇄adjustNoxious PlagueNow upgraded with Aghanim's Scepter
▲buffNoxious PlagueDecreases cooldown by 35s. Reduces Magic Resistance of affected units by 20% and allows additional spreads to deal initial damage
▲buffTalentsLevel 15 Talent Poison Sting Slow increased from +5% to +7%