Currently infection does this: 
In game this means that after inflicting the status effect completely, enemies take more plague damage e.g. plague balls in my tents inflict 16 damage normally and when the enemy is infected they inflict 16+3 damage (the additional damage increases based on the damage of the skill so I assume that it is a simple percentage bonus)
The interaction with fire is either buggy or the values are so low that they are not noticeable in the early game, More on this here: Fire and Infection do not synergize as intended
Unlike fire, the bonus damage only comes after you inflict the status effect, after which you have to deal a good amount of damage to actually gain an advantage from the bonus damage, this makes plague damage only useful against enemies with a lot of health.
Healing reduction is basically an effect that only comes into play in rare circumstances and usually only matters when the player is the one suffering the status effect.
But the real problem is that dealing plague damage has no substantial gameplay difference compared to dealing physical damage. The only difference is that you deal more damage to large enemies and less to everyone else (im not counting the enchantments).
Rework Idea:
When an enemy is infected they cough for 1 sec (thus stunning them for 1 sec) after which they take additional damage from all sources, and leave behind a trail of plague that infects (over time) whoever breathes it.
Dealing fire damage to an infected enemy cures the infection but inflicts a fully stacked burn effect and the trail also burns dealing a small burst of fire damage to anyone inside.