I'm running a 4e D&D campaign, and want to introduce a plague that the players must stop (or watch the world die around them), but it needs some fleshing out.
The Photophobic Plague is actually an insidious parasite that was crossed with a hemorrhagic fever, making the parasites into carriers to deliver the biological payload.
Photobic Flu Parasite:
The parasite is born small, nearly as large as a grain of rice. The creature has four distinct legs, each with a flat 'foot', intended mostly for swimming. The parasites naturally move through ecosystems fecal-orally, through contaminated water, and through natural predation. During the initial infancy stage of life, the parasite seeks only a host to inhabit.
Upon entering a host, the parasite moves towards the optic nerve, attaching onto it. The parasite initially feeds off the electrical pulses traveling down the nerve, until reaching maturity, when it starts producing young. The young move to exit the body and inhabit their own host. The blind are oftentimes immune to Photobic Flu Parsite, for their optic nerve cannot sustain them through the birthing process.
At this point, the host has intense photophobia, is experiencing flu-like symptoms, and has trouble standing. The parasite usually survives up to two weeks before perishing.
Extremely bright light can cause the optic nerve to become active enough to kill the parasite. Getting a patient to endure the light while photophobia is in full effect can be seen as cruel however.
Magic can be use to exorcise the creature, kill it, or nullify it.
Otherwise, the body's own immune system will usually kick in after a week or two.
Blister Blight is an aggressive hemorrhagic fever, spreading through water droplets exclusively. The blight begins with a violent fever that lasts a few days, before disappearing up to a week. During this time the afflicted individual is most infectious.
After the respite from fever, blisters, boils and lesions start appearing on the body. Mucus membranes begin to bleed (Nose, Eyes, Genitals, Mouth, Lips), leading to a rapid painful death.
Treatments: Immediate clerical or other aggressive healing.
Its only route being water droplets and its quick lethality make Blister Blight outbreaks small, contained and fast. The disease cannot spread far before killing its hosts.
Blister Blight, upon being crossed with the Photobic Flu Parasite increased the infectiousness of the Blight, allowing its use as a weapon. The photophobic effects serve as an early warning, however, and the Blight relies singularly on the parasite to continue production of the blight pathogens.
Symptoms (In rough order of appearance):
Fever, Boils, Lesions, Blisters
Photobic Flu Parasite
Removal of the parasite from the optic nerve increases survival chances greatly because the blight no longer relies on the host (where host is the sap infected with the parasite) to continue production of the blight.
Clerical Healing can meet success if the parasite is removed.
I intend to have my players meet the man responsible for crossing the parasite with the blight- after those in the group who are inclined to drink too much contract the Photobic Flu of course. His work will be slowly revealed to them (first the lab where they discover the work he did to isolate the parasite, then where his first attempts to cross the two happened, finally they have to stop him from releasing the plague), causing a mad dash to stave off pestilence.