Long before Resident Evil became a first-person horror series, it began as a third-person survival horror franchise. The series primarily focused on surviving an outbreak of mutants and zombies born of the Tyrant Virus. The t-Virus is an offshoot of the Progenitor Virus, which gave Albert Wesker his abilities. The Umbrella Corporation created the Tyrant Strain to replace standing armies and build revenue to continue funding the Wesker Project. Unfortunately, this meant creating a highly contagious virus with a 100% mortality rate. What came was something even more horrifying.
What makes the virus so interesting is how it isn't only one strain. In fact, it has since turned into multiple strains that create many other reactions within its hosts, fundamentally changing them for the worse. While there are many iterations, the most iconic version leads to the zombies found in the original game. However, this version is most effective thanks to a real-life virus that is highly contagious and incredibly deadly in its own right.
In the late 1970s, Dr. James Marcus continued the year-long project that would become the t-Virus. However, his work eventually leads to a breakthrough becoming the catalyst that gives the virus its now infamous name. When infected with Marcus' version of the virus, the host suffers violent symptoms, including increased aggression and memory loss. However, its most volatile symptom is necrosis of the flesh that begins as an itch and ends with the host taking off chunks of their own skin. Ultimately, Marcus successfully creates the franchise's zombie virus.
While effective in every sense, his protégé, Dr. William Birkin, takes the research one step further after stealing it. Melding the disease with the Ebola Virus, Birkin successfully makes it highly contagious while ensuring the survival of the zombified hosts. Thus, the virus proved itself an effective killer, but there was still the glaring issue that it couldn't infect 100% of the population. As a result, the t-Virus underwent even more experiments to compensate for the ten percent that could not be infected.
One of the more Frankenstein-esque modifications to the virus involves the t-Virus melding with a human egg and reptile DNA to create the Bio-Organic Weapon known as the Hunter. The Hunter's main task is to hunt down and kill the remaining 10% that isn't infected. From this experiment, a new strain of the virus known as the Epsilon strain is also born and leads to mutants like the Lickers and Crimson Heads.
Even though the t-Virus is deceptively simple in creation and execution, its effects are anything but. Its transmission is even more simple, with infection being born through injection, water, air, bites and scratches. However, while the zombie aspect of the virus is deadly in its own right, the addition of Ebola fundamentally changes everything about it. Thanks to the real-life disease, the t-Virus helps create an even deadlier disease that eventually gives birth to some of the series' most dangerous mutants.