Infects the victim with harmful poison. The more the target has been injured the more damage Viperish will inflict. If the Dryad carries a shrunken head it will increase the chance that opponents of the same type will also catch the disease.
As of patch 2.34, this spell has a tremendous exploit when modified in a specific way. With the Relapse (duration) bronze mod, the Transmission (enhanced spread) silver mod, and Contaminate (range) modifictions, it is very easy to get the spell's effect to travel very far off-screen from the character, and can kill great numbers of monsters in a very short time. This effect can happen with other combinations of modifications, but it seems to be most powerful when these specific modifications are chosen and when the character has the specific shrunken head of surrounding monsters equipped.
While this can be used to obtain large amounts of experience in a very short time, there is the problem of obtaining items and money dropped by monsters that have died far away from the character; most of the loot will rot long before the character can locate it. Since the spell's effect travels in an expanding wave around where the outbreak began, the vast majority of loot will inevitably be wasted.
This exploit continues to work well into Niobium difficulty, and in fact becomes even more powerful in contrast to other combat arts as difficulty goes up, since it inflicts damage by a percentage of the infected monster's current health. It appears to get even easier to start an epidemic at later levels with the wrong head equipped, possibly due to a hidden mechanism that depends on character levels or skill levels.
This exploit has been reported to Ascaron as a multiplayer server bug, as the calculation of hundreds of monsters transmitting the disease back and forth causes very noticeable overhead and can make the client unresponsive. It would be a major breach of etiquette to abuse this technique on a multiplayer server.
The Community Patch significantly nerfs the duration, range, and chance to spread of Viperish Disease in order to avoid this exploit. However, in return, the CM Patch also grants some flat poison damage to the spell. This improves the spell's raw killing power and allows the damage to be boosted by skills, attributes, and item modifiers.
Pros and Cons
Can spread far and fast
No way to boost the damage - no skills, attributes, or item modifiers will affect Viperish Disease (requires Community Patch to fix).
Important note: This spell has no effect on creatures of type Energy (ghosts, fey fires) and some kinds of undead, notably those found in the lizardman swamps.
Viperish Disease inflicts a special type of damage (called "Plague Poison" in the game files) that is neither normal Poison damage nor Poison DoT, despite the word "poison" in the CA description. Therefore it is not boosted by +Damage modifiers and is not resisted by Armor.
The spell FX for Viperish Disease has been bugged since release, resulting in the initial target being surrounded with overlapping green squares (see Single Infection picture below). This visual bug was finally fixed in the Community Patch 1.60.