Inflicts poison on enemies who make contact with Isaac.
The poison deals 2 damage per tick (3 damage per second) and lasts for 2-3 ticks.
The poison deals Isaac's damage per tick (1.5x Isaac's damage per second) and lasts for 2-3 ticks.
The poison deals Isaac's damage per tick (1.5x Isaac's damage per second) and lasts for 6 ticks for normal enemies and 3 ticks for bosses.
The poison lasts for 6-7 ticks and deals damage equal to (0.2*HeartContainers^2) + 2, where HeartContainers is the total number of heart containers and Bone Hearts Isaac has.
The poison can be continuously applied to bosses, which normally are immune to status effects for a brief time after being inflicted with one.
Lets Isaac deal 12 contact damage per tick on top of applying poison (24 damage per second).
Enemies who are killed after they have been inflicted with this type of poison have a 20% chance to drop Black Hearts.
The black heart is only dropped if the poison was caused by The Virus. The item's effect doesn't extend to poison tears.
If an enemy doesn't do contact damage, The Virus can safely be applied to it for more chances at black hearts. Flies, Mulligans, and Hosts are the most common enemies this can be applied to.
With Skatole, Hives and Swarmers will constantly spawn normal Flies, meaning they can be endless sources of Black Hearts.
Although Stonies and Shopkeepers can't be visibly poisoned, they still can drop Black Hearts if destroyed after being touched.
Enemies won't start taking poison damage if they are pushing up against Isaac; The Virus will be constantly trying to apply poison and refreshing the ticks where poison damage would occur.
Gnawed Leaf and The Virus can still work if Isaac has a short-range orbital such as Cube of Meat or Ball of Bandages, as it will kill enemies pushing against Isaac that are not taking poison damage while still dropping Black Hearts.
Synergies
Abyss: When Isaac uses Abyss to absorb The Virus it turns into a green locust that poisons enemies.
The poison deals 2 damage per tick (3 damage per second) and lasts for 2-3 ticks.
The poison deals Isaac's damage per tick (1.5x Isaac's damage per second) and lasts for 2-3 ticks.
The poison deals Isaac's damage per tick (1.5x Isaac's damage per second) and lasts for 6 ticks for normal enemies and 3 ticks for bosses.
The poison lasts for 6-7 ticks and deals damage equal to 
Bone Hearts Isaac has.
Enemies who are killed after they have been inflicted with this type of poison have a 20% chance to drop 
Black Hearts.
-0.1 speed down.
This item belongs to the 
Spun set. Collecting three items from this set will transform Isaac into a buff berserker.
Lust.
Flies, 
Mulligans, and 
Hosts are the most common enemies this can be applied to.
Skatole, 
Hives and 
Swarmers will constantly spawn normal Flies, meaning they can be endless sources of Black Hearts.
Although 
Stonies and 
Shopkeepers can't be visibly poisoned, they still can drop Black Hearts if destroyed after being touched.
Gnawed Leaf and The Virus can still work if Isaac has a short-range orbital such as 
Cube of Meat or 
Ball of Bandages, as it will kill enemies pushing against Isaac that are not taking poison damage while still dropping Black Hearts.
Abyss: When Isaac uses Abyss to absorb The Virus it turns into a green locust that poisons enemies.
Odd Mushroom: The speed down from the Virus does not stack with the Odd Mushroom; if both are collected, only a total of -0.1 applies.
Little Baggy - Pick up any 2 of the following items in a single run:
Roid Rage, 
The Virus, 
Growth Hormones, 
Experimental Treatment, and 
Speed Ball (
Syringes 
The Binding of Isaac: Rebirth
