The Sweet Tooth virus' origins are now known, but before this happened, the real source of the H5G9 virus was heavily foreshadowed in Netflix's sci-fi series. The show is based on the Vertigo comics of the same name by Jeff Lemire, which did eventually reveal the origins of the sickness and the hybrids, parts of which were loosely adapted in Sweet Tooth season 2. In the show's second season, the Sweet Tooth Birdie plot in Alaska and Gus' misadventures in post-apocalyptic America came together to reveal how exactly the H5G9 and the hybrids were created.

https://screenrant.com/sweet-tooth-theory-virus-hybrids-cause-origin/