It increases the damage on the skill by 40%. You can test it yourself, since it updates the main skill tooltip once you put a point in there. The life reduction part won’t update on the skill tooltip though.
You have to turn “gore” of in the game options for Bloody Pox, because enemies that gib won´t spread the disease. With gore off, the skills does noticeable more dmg over all.
This is kinda a bug, but it´s not gonna be fixed. “The more you know”.
Currently leveling a cabalist with Pox/Fevered Rage. DMG is off the charts.
Use GrimInternals DPS meter for more accurate info about dmg dealt.