I was avoiding going into a long spiel about the campaign and the circumstances because I figured no one would want to read a bunch of background. But it seems you can never give too much information when asking for help.
To answer Coyraven and LostSoul:
The country of Arn established a colony six years ago on this new continent. It was an endeaver by several noble houses to explore/look for raw materials/etc. A year after the colony landed, all communications was lost and boats stopped returning. Until now, no real effort was made to re-establish with it, but now the Houses want to find out what happened to their investments, and try again.
Two of the PCs are members of one of the Houses, one is looking for pearls, and another (who may not show; total flake) was given a vision by his God to go (his god is Yog Sothoth, who has some interests in the continent).
With them are about a hundred and fifty others (other house reps, laborors, healers, etc). The PCs are one of the few Adventurers who are going to do the sweeping/cleaning/exploring of the colony and area.
The party right now has dropped anchor beside a small island a rowboat's distance from the shore where the colony is plunked down. This is their "base camp", where they will execute a launching point. Supplies will be dumped on a barrier island (basically a big blob of silt and sand a dozen feet off the river's mouth).
The PCs aren't going to jump in immediately; they are camping out at the base camp right now, and waiting until dawn. I plan to have the demons summon a "Mist" from the statue; the mist will have several Mist demons, and will offer cover for a wave of zombies they send in. The zombies are from the dead hosts who have been killed for whatever reason, and are thus unusable to them.
-The environment has begun to become infected by the virus as well. So jungle plants and veins have mutated and altered, some have even begun to grasp some level of intelligence; wheezing and heaving, whispering in your ears.
-The mouth of the river has become full of swamp plants and a layer of scum similar to human skin.
-The buildings of the colony are made of stone, held-together by plant matter and infected material.
-The bones of those rotted away are used to decorate the colony.
I like these. The reason I'm not using the other bits is because the virus doesn't work in dead flesh; it's spread via speech, and is intended to destroy a host's will, so that they are ripe for possession.
Since it might just offer some more help, I'll go ahead and give some more detail.
The Virus's Origin
There once was a city of beautiful amazons. They were the most fetching entities all around, some say endowed with fey blood. But they grew too prideful in their looks, and their hubris called down a curse. The women were twisted, made into bird-like, craven seducers, their beauty deadly because they were driven to lay their eggs in the bellies of men (I.e. the origin of Harpies).
The rulers of the amazons sought to reverse this curse, and turned to a demon prince (Demogorgon most likely), seeking help, trying to find a way for them "To be loved and revered again". The Prince responded by unleashing the virus. It lead to widespread possession; anyone the harpies came into contact with, any time they tried to sing, or wail their desperation, those would be infected, then possessed, and the demons inhabiting the ugly, tumor-ridden hosts would "revere" the once-great amazons.
Most of the amazons fled in horror. Soon, the surrounding inhabitants in the jungle waged a siege against the amazon's city, sundering it, and confining the virus and the Amazon Queen and Priests to their temple fortress. The Queen, her retainers, and the demons that they had spawned were trapped. The queen's sorrow blighted the land around their prison, turning it to barren desert. THe natives managed to contain this blight, but it has left a swath of sand that leaves no footprints around the temple palace.
Five years ago, the Arnian explorers opened the palace. They were exposed to the virus, and were beaten back by the inhabitants. When they feld back to the colony, they unwittingly spread the virus.
For the PCs to "cure" the virus, they're going to have to go into the Temple Palace, and destroy the source. A macguffin conveniently tied to the palace and the Queen.