What Causes “The Sick” in 'Sweet Tooth'? Season 3 Reveals the Cause and Cure

Spoiler alert: This article contains spoilers for Season 3 of Sweet Tooth.

Ever since Sweet Tooth officially landed on Netflix, fans worldwide have been enthralled by the dystopian future story where half-human, half-animal beings roam and the population is infected with a terrifying virus known simply as "The Sick."

This virus, referred to by in-show scientists as H5G9, which later evolves into the doomsday strand, is certainly not something to scoff at. In the world of Sweet Tooth, the virus has the power to bring global civilization to its knees and it does.

The real world has become no stranger to powerful viruses running rampant and upending society, but nothing is quite like what Sweet Tooth depicts in their own viral pandemic. So, what exactly causes "The Sick" and do Gus and his crew ever find the cure? We've got answers!

The disease in Sweet Tooth, known as "The Sick," has largely wiped out humanity and society by the time that viewers are shot into its world. Beyond the dwindling number of actual humans, new hybrid people that are half-animal, half-human have emerged as well and are regarded by many as a problem. Individuals and organizations hunt them down and kill them because they see them as some sort of threat.

So, is "The Sick" real? Definitely not. There really isn't any plausible scientific case for a virus of this size and power to actually take shape on our planet, but the face-value symptoms of the show's H5G9 do mimic a lot of known viruses, specifically COVID-19.

Symptoms such as coughing, fever, and difficulty breathing definitely overlap with real-world viruses, but the notion that it would culminate in a world also inhabited by hybrid babies is purely a work of fiction.

Interestingly enough, when Sweet Tooth initially debuted as a comic book series back in 2009, the virus that destroyed humanity was known only as "The Sick." This means that the creator and/or Netflix writers took some creative liberties in the television adaptation by naming the virus H5G9.

Whether or not this decision was made to align with recent real-world events and provide another layer of spookiness to the storyline has yet to be confirmed.

In Season 3 of Sweet Tooth, viewers learn that the virus's origins actually have a mythical background, not a scientific one as initially believed. After traveling to the Arctic in 1911 in search of a cure for all illnesses, a scientist named Dr. James Thacker stumbled upon a holy cave and befriended a local native tribe. However, during his visit, a crew member named Nathaniel Burke who had been traveling with Thacker forged a bond with one of the natives — Ikiaq — and she became pregnant.

Things began taking a turn for the worse after a greedy Thacker came across a tree inside the cave, that once cut, would spill the blood of the Earth. Thacker took a sample and quickly learned he had located exactly what he had traveled to the Arctic for — a cure for all illnesses. However, his discovery didn't come without a price. Because Thacker took something that didn't belong to him and wanted more, his actions not only released The Sick, but subsequently triggered the birth of the first hybrid.

Shortly after Thacker's discovery, his crew began to experience the first telltale symptom of The Sick: a twitching pinky. While Thacker planned to share his newfound information with the rest of the world, his crew decided to kill themselves off by ingesting poison to try and prevent the spread of the ungodly disease Thacker had awakened.

Ironically, Thacker met his demise immediately after, as his exposure and ignition of the disease led to the emergence of the first (and largest) purple flower blossoming from his chest.

While Ikiaq was left behind, she also died from The Sick and left behind a half-man, half-caribou baby named Munaqsriri who would later become nothing more than a mythical creature people would refer to as the "Caribou Man." After Ikiaq died, the events that transpired were largely covered up by history and the cave was soon forgotten. The Sick had been buried, for now.

Fast forward a little over a century, and Fort Smith stumbles upon the same cave and disturbs the same remains. In doing so, Dr. Gertrude Miller, aka Birdie (Amy Seimetz) and Gillian Washington (Robyn Malcolm), also Thacker's great-granddaughter, unleash the virus yet again, this time wreaking havoc on the modern world and giving birth to the likes of Gus, the show's protagonist.

Initially, Dr. Aditya Singh (played by Adeel Akhtar) believes that sacrificing Gus is necessary to cure The Sick. However, it becomes evident that it's more than just some of Gus's blood needed to end virus. Although Gus did offer a blood sacrifice, it was the act of burning down the tree—ironically bearing branches resembling deer antlers—that ultimately led to the cure of The Sick.

So, although Dr. Singh and even the Last Men's theory that hybrids can cure The Sick was technically accurate, as Gus was the one to find a cure, it wasn't by killing them. But, as Munaqsriri explained it, it's the people that are the disease and The Sick is actually the cure.

While Gus and his team successfully cure The Sick before it claims the lives of Jepp (Nonso Anozie), Becky (Stefania LaVie Owen), and a few others, it's presumed that only a handful of humans survived the Doomsday Strain before the cure was discovered. These survivors were able to grow old without the threat of the deadly virus.

Seasons 1–3 of Sweet Tooth are streaming now on Netflix.