Andrew Lincoln's character was written out of the series and moved onto a cinematic spin-off. Although the zombie virus is obviously central to The Walking Dead's premise, the cause of the outbreak has never been revealed. Fans have an approximation of how the infection behaves and, in a unique twist, The Walking Dead's zombie virus lies dormant within everyone just waiting to be activated. Nevertheless, the origins of the virus are deliberately unexplored, and Kirkman has only ever joked about how his fictional apocalypse began.
Numerous fascinating Walking Dead theories have emerged over the years. From "Old Man Carl" - which suggested the entire story was being told by an older Carl Grimes - to the series being one big coma dream in Rick's mind, fans had plenty of ideas about The Walking Dead's secrets. In 2020, however, such theories are few and far between. Kirkman's comic series has ended (disproving the Rick coma theory but sort of acknowledging Old Man Carl) and the AMC TV show is rapidly heading towards its eleventh season, and will soon overtake its source material. The biggest topic fans can theorize over now is the mysterious helicopter group known as the CRM.
However, there is one intriguing, long-standing The Walking Dead theory that could potentially still be proven true on TV: Rick Grimes being immortal. This concept has been around more or less since The Walking Dead's popularity first blew up and various different strains of the theory exist, one of the most popular coming from inexplorata on Reddit. But as crazy as Rick being immortal might sound, there is some evidence (albeit mostly circumstantial) to the idea.
There are various theories suggesting Rick could be immune to the zombie virus; this has been conclusively disproved in the comics and remains untested on-screen. The immortality theory goes several steps further and relates to the origins of the outbreak. Since its beginnings are unknown, it's entirely possible that The Walking Dead's virus could be man-made. But what would the motivation be to create a pathogen that brings back the dead? Perhaps the most obvious answer to this question is immortality - to create a biological solution to death. Given the state of the world in The Walking Dead, the program obviously didn't go to plan, with subjects defying death, but at the expense of being turned into mindless flesh-eaters. Whoever was behind these experiments aimed to cheat death, and somehow ended up making it much, much worse.
The second part of the theory posits that if a failed immortality experiment caused the outbreak, could the virus have actually worked on a select few - Rick Grimes, for example? Again, this is a non-starter in the comic books where Rick turned into a zombie like everyone else, but the TV series could absolutely head in a different direction. Rick survives the potentially fatal gunshot wound that first puts him into a coma in The Walking Dead season 1, and years later lives through being impaled and blown up before his eventual rescue. While these miraculous recoveries could be down to plain old protagonist plot-armor, is something more biological at play? If the virus was supposed to grant immortality, perhaps it had the intended effect on a small percentage of the population, explaining how Rick constantly survives and justifying the aura of mysticism The Walking Dead wraps around its former lead character. The new world needs Rick Grimes, after all.
Considering how The Walking Dead valiantly tries to ground its zombie apocalypse in reality, Rick being immortal is incredibly unlikely. It could also be argued that an immortal Rick wouldn't wound so easily, or may not show signs of aging. Furthermore, Rick couldn't be the only person in the world that the immortality virus worked properly on, so why hasn't any other character in the franchise shown the same death-defying abilities? Rick Grimes being immortal is a fun and interesting theory, and may yet be proven true in The Walking Dead's future, but it would be a tonal and narrative leap that most fans would struggle to accept.
The Walking Dead season 10 is currently on hiatus.