“Sweet Tooth” definitely establishes H5G9 as a disease worth fearing, but that doesn’t mean it’s one that could ever actually exist. Many of the symptoms of H5G9 mirror actual diseases, and COVID-19 in particular. The main symptoms include having trouble breathing, coughing, and running a fever, but the overlap between H5G9 and COVID-19 stops there. The idea that a virus could create a world partially populated by hybrid humans is pure fiction, and it’s a result of the specific circumstances that “Sweet Tooth” offers for how “the Sick” came to be.
As the series explains, the H5G9 virus origin story starts in 1911, when a physician-in-training named James Thacker stumbles into a holy cave after befriending a local tribe. Thacker ultimately disturbs some holy remains, thereby unleashing the virus and the hybrid children simultaneously.
The native populations are wiped out almost immediately, but the events are eventually covered up and forgotten by history. More than 100 years later, a man named Fort Smith stumbles upon the same cave and disturbs the same remains, ultimately unleashing the virus on the modern world.