To add details to the lore. Maybe Scorpion-7 is a Scientific research corporation that was developing inter-dimensional travel in one project and fulfilling a bioweapon contract at the same time. Scorpion-7 discovers the Excursion dimension, they find a watered down Turned disease which could be a virus, fungi, prion, or even a disease type that was previously unknown to humanity. They would conduct experiments with this disease. At first it would affect the wildlife of what was the Excursion dimension and eventually make the jump to humans.
There would be more expeditions into the Excursion to get more samples of the disease under the Scorpion-7 project called “Project Charon”, named for the ferryman of Hades in Ancient Greek mythology. The inter-dimensional travel project would intersect with the bioweapon project once they brought back the disease of interest. The Scorpion-7 Employees working on Project Charon would eventually get their hands on some samples of this disease. They’d test it on animals from the Excursion and with the constant meddling of Scorpion-7 in the Excursion, the dimension gets corrupted and leads to the creation of the Apex.
The disease eventually mutates and makes the jump to humans in a Scorpion-7 facility. This unexpected mutation leads to the creation of what we know to be the Turned. There’s no explanation for how the spores came to be at this moment so I’d most likely scrap the spores and would probably put in something else. Remember, we’re dealing with another dimension here and the laws of physics in the Excursion would most likely be very different than our own universe, so we can be creative here. At first, the outbreak is contained but a series of mutations allows the disease to spread quicker. This disease would most likely not be fungal but rather would require a whole new disease category of it’s own due to the ability to effect to mutate an individual’s genetic code at an accelerated rate.
Just for some thought on the origins of the disease. Scorpion-7 can of course be replaced by another name such as the Apex Corporation for example.