One could forgive the Internet Take Industrial Complex for taking Kirkman’s answer and running with it though. Kirkman has a decent track record of spoiler-reveals. Through 193 issues of The Walking Dead, he was fond of holding spoiler-y information over readers’ heads in his Letter Hacks Q&A sessions at the back of the book. Not only that, but Kirkman was once a contestant on Comedy Central’s @midnight and when host Chris Hardwick cheekily asked for an upcoming Walking Dead spoiler, Kirkman just flat out revealed that one day a character named Negan will arrive to kill Glenn on the show, several seasons ahead of the actual moment.
Alas, this Twitter exchange was one of those instances where Kirkman really was joking. He confirmed as much in April via his account.
In reality, Kirkman has always been adamant that the origin of the zombie pandemic on The Walking Dead would never be explained. And in the end, the writer was true to his word. The Walking Dead concluded its run with Issue 193 last year and nowhere in those 193 issues is there any indication of what kind of virus or supernatural event could have reanimated corpses around the world.
For years Kirkman steadily maintained that the cause of the zombie outbreak was never important. He was more concerned with telling a simple horror (and later action/adventure) story with relatable characters. Zombies were inherent to that mission as antagonists and occasional environmental obstacles.
The closest Kirkman ever came to going on record in describing what caused the zombie outbreak was in the Letter Hacks section for Issue 147. In it, he explained just how someone becomes a zombie thusly: