Fortinbras Plague

The Fortinbras Plague is a highly virulent contagion that threatens to kill everyone aboard ark Keelah Si'yah 30 years prior to its scheduled arrival in the Andromeda galaxy.

Properties
Named after the character in Hamlet because it "always swarms in the end and destroys it all" by elcor doctor (and Shakespeare fan) Yorrik, Fortinbras is an artificial chimera virus assembled with traits from multiple diseases plaguing Council and non-Council species alike. The highly mutagenic virus is classified by the doctor as belonging to the Metastolizomai family and can infect all known species it comes across. It is transmissible through both air and contact with contaminated material, with an R-nought of 22 to 26.

Its main shape comes from the infectious volus disease Yoqtan, although its payload has a number of RNA sequences from Asari Cyanophage, Ayalon B, Titan's Tears, and measles. A smaller part of its genome is a "trash compactor" of dormant/junk RNA from a dozen sources: Kepral's Syndrome, Varren Scale-Itch, bubonic plague, even Ardat-Yakshi.

The symptoms are roughly the same across infected species. On an observed batarian victim, initial signs of the disease include copious projectile vomiting, high fever, coughs, chills, and nausea. Drell corpses are shown to have swollen tongues, possibly inherited from Yoqtan. Three hours after confirmed infection, bluish sores (turquoise in hanar) appear on the host's skin, which may collapse into fistulas and cause leaking of internal fluids. The subject becomes ravenously hungry as well.

In the terminal stages of the disease, victims begin to hallucinate and lose control of their cognitive functions, usually manifesting in insanity or irrational rage. Around 18 hours after observable infection, the subject expires. This applies to batarian victim Jalosk Dal'Virra; elcor infectees persevere for days (accounting for precautions taken) before succumbing.

Because Fortinbras has so many exotic proteins to draw on it practically becomes a different virus every generation: sometimes slightly, sometimes drastically. Deleterious mutations are quickly discarded, but successful ones succeed thoroughly. This has led to initial difficulties in synthesizing a cure.

Background
Created by Keelah Si'yah's own captain Qetsi'Olam vas Keelah Si'yah, Fortinbras was smuggled into the ship by way of canisters ostensibly scented with usharet flowers from Rakhana. These were placed around the cryopods of drell Qetsi deemed most likely to spread the plague: the most outgoing and gregarious personalities.

About 150 years prior to the ark's current predicament, Qetsi sneakily revived herself and activated the contagion, relying on her accomplice Malak'Rafa vas Keelah Si'yah's computer worm to fool ship systems into thinking nothing is going on. However, the worm malfunctioned and let cryo temperatures rise a bit above standard, leading to the virus very slowly replicating and mutating unabated within its hosts for decades.

Fortinbras was designed to be a carrier virus whose hosts did not show any symptoms. It was supposed to be harmless except for its intended targets the Council races: humans, asari, turians, and salarians, aimed at reducing their populations in a bid to secure the minor races' ascendancy in the new galaxy, with the quarians preferably on top. During the shift of Sleepwalker Team Yellow-9 50 years prior to the outbreak, the drell infectee Soval Raxios came into contact with representatives of all races aboard the ark, leading to more opportunities for Fortinbras to develop.

Because drell lungs were poor infection vectors, Fortinbras adapted to target the host's brain. The strain was already lethal by the time Soval contaminated the others, resulting in the crisis Sleepwalker Team Blue-7 was woken to contend with.

Mass Effect: Annihilation
The computer worm prematurely wakes over a thousand colonists from cryosleep during the emergency, drastically increasing the number of potential infections and rendering quarantine efforts moot. Fortinbras even passes onto some volus and quarians in spite of their suits; volus suits not built for disease filtration in the first place, and quarian suits due to the race wearing them even in cryo and centuries of stasis weakening them enough for points of entry to appear.

After days of identifying and analyzing the virus with limited resources due to the computer worm rendering most tech useless, Yorrik arrives at a possible solution that requires eezo and a naturally immune subject. The eezo could "teach" a copy of the original virus to infect cells like normal, then destroy the host's copies in the same way as an immune person does. The retrovirus would leave the original virus dormant at least and purged at best.

With the eezo supplied by batarian smuggler Borbala Ferank and the shipwide search for an immune person turning out to be her own captain Qetsi, Yorrik creates a retrovirus hypospray sufficient for one person only. To atone for the death toll, Qetsi injects the retrovirus into herself and goes to all six racial sections of the ship, without her suit to maximize exposure, finally dying inside the high-pressure volus section trying to spread the cure to its inhabitants.

The retrovirus succeeds in its purpose. When the chaos finally dies down, after attending to their dead the wakened survivors return to their cryopods to sleep off the last 30 years of their journey.