Green Leech virus disease 2185 (GREVID-2185), also simply known as Green Leech and the Gozu virus, was a contagious artificially-engineered disease caused by a virus, the Green Leech-1. The first and only known case was identified in the Gozu District on Omega in July 2185 CE, where it quickly spread throughout the entire district, infecting nearly 14,000 people and claiming the lives of 7,300 victims: in the midst of this outbreak was the Gozu Mercenary War, a conflict between the Blue Suns providing protection for the district and an invading rogue cell of the Blood Pack consisting entirely of renegade vorcha who deliberately spread the disease to help them quickly conquer the district. The disease in reality was a bio-engineered virus created by the Collectors for use in expanding their abductions of human colonies to include non-human centric populations, allowing them to extract human victims from colonies with a large number of non-humans present. To this degree, the virus was designed to infect and kill any and all known species with the exception of vorcha and humans, with the former being immune to all disease while the latter were a targeted exception. In July 2185 CE, the Collectors chose the Gozu District of Omega as a controlled and isolated environment in which to test this new weapon, using a renegade vorcha cell of the Blood Pack as a convenient "mule" in which to spread the disease for them.

The symptoms of the Green Leech virus are variable but often include fever, headache, cough, breathing difficulties, loss of smell, loss of taste, slurring of speech, uncontrollable twitching, insanity, spontaneous hemophilia, development of lesions and blisters, hallucinations, temporary to permanent blindness, phobia towards water, and eventually death. The virus goes through three stages: Stage 1 is subtle, producing a mild headache, loss of smell and taste, and a shortness of breath. Stage 2 becomes more severe, producing stronger headaches, breathing difficulties, twitching, hallucinations and the development of blisters and lesions. Stage 3 is irreversible, but have the most variable symptoms: some victims at this stage are driven insane and suffer from extreme hallucinations, having a phobia towards water that is similar to rabies, and suffering from uncontrollable twitching in their limbs, with tremors leaving their body unable to sit still. Other victims suffer from temporary or permanent blindness, their lesions and blisters grow more severe and cover parts of their body, they develop spontaneous hemophilia where their blood does not cut properly and bleeds profusely from cuts and wounds, their speech becomes slurred and unintelligible, and they develop an intense fever. Regardless of how Stage 3 develops for the victim the end result is always the same: death. Worse still was that the disease, with the exception of the immune vorcha and humans, who were deliberately not-targeted, was communicable across all known species, with it able to infect asari, turians, salarians, krogan, batarians, elcor, quarians, volus, hanar and more. Its fatality rate was 92%, making it an extremely efficient and ruthless disease, and those infected were almost certainly guaranteed to die with very little exception, with various treatments and attempted vaccines failing to halt or cure the disease, at least until Mordin Solus finally developed a cure on 11 August 2185 CE. The disease ruthlessly targeted and destroyed a victims' respiratory system, doing so with aggressive speed and precision, with most victims transitioning from Stage 1 to 2 in a matter of hours, while Stage 2 and 3 was a matter of days. It was also incredibly infectious, spreading throughout the Gozu District and claiming hundreds of lives within the first week. Those who managed to survive Stage 3 or received Mordin Solus' cure were left permanently crippled by the virus in one way or another, either left a vegetable, losing control of one limb or another, suffering with breathing difficulties for the rest of their lives, having permanent exterior deformities such as lesions and blisters, and more. Some survivors reported having mild hallucinations after receiving the cure, while others suffered from intense migraines every once and a while. The more unfortunate ones were cured so late into their infection that they had to be euthanized, barely able to live without considerable effort. Those with more mild after effects, such as permanent loss of smell, sight or taste, were considered the lucky ones.

GREVID-2185 was transmitted via numerous vectors. Designed to be as infectious as possible, the disease was initially spread deliberately by Maw's Blood Pack, using vials to poison the district's water supply or, in some cases, deliberately infecting themselves with it, acting as carriers before spreading it through contact. While no airborne version existed at the time, as the Collectors didn't want the disease to spread beyond the district in order to avoid attracting any larger scale attention, one would have likely been developed following a successful test in the Gozu district. Once enough victims were infected, the disease spread through fluidic and touch-based contact, with the especially unsanitary conditions of the district making it perfect conditions for the disease to spread quickly. While Aria T'Loak's quarantine of the district proved effective in isolating the disease and stopping its spread beyond Gozu, actual treatment of the disease prior to the development of a cure involved destroying the bodies of the infected, with the Blue Suns heaping bodies into piles in the streets and burning them. Detecting the disease proved extremely difficult, as it often manifested initially as common symptoms such as a cold or flu, and by the time Stage 2 came around and the symptoms became more obvious, it was often too late to isolate the victim. As such, GREVID-2185 were immediately "assumed" to be infected if they so much as coughed or sneezed, and while Mordin's clinic tried their best to treat these people, the Blue Suns took to more extreme measures, either locking people inside their apartments or shops, or, with the more twitchy and paranoid mercs, shooting the victim dead on the spot.

Regardless of the virus' efficient transmission and infection rate, as well as its near-total mortality rate, it never spread beyond Gozu and Mordin Solus was able to finally develop a cure after a month, which the Normandy Squad proceeded to deploy after recapturing the district's ventilation control facility and using its facilities to deploy the cure via airborne transmission throughout the rest of the district, eliminating the virus in its entirety: vials of the cure were stolen by Daniel Abrams prior to this and taken to be spread into the district's water supply, but many survivors who were blaming humans for spreading the disease saw this as proof of human involvement and one particular group of batarian victims almost killed Abrams until the Normandy Squad arrived and rescued him. Within a day the disease was completely eliminated and on 13 August Aria T'Loak officially lifted the quarantine. The Gozu District was the only known deployment of the virus, as the Collectors quickly cancelled its use, as Cerberus now possessed a cure for it, thus making it useless. All remaining samples of the virus that remained on the Collector Base were destroyed with the rest of the Base when its reactor was overloaded at the end of the Collector Base Assault in October of the same year.