As far as I know the whole zombie genre was started by Richard Matheson, with his novel I Am Legend. There has been three movie versions of it, one with Vincent Prince, one with Charlton Heston, and the last with Will Smith.
Art imitates life, or as Erza Pound wrote, "The artist is the antenna of the human race."
So what is this antenna telling us?
Zombies are parasites. And we have a lot of parasites today, ones who are in fact going to bring this country down. The poor are parasites on taxpayer money, but the real bloodsuckers are those one percent who are sucking the life out of the nation. What they both have in common are the government, and the government is out of control. So out of control that it can no longer be contained.
I used to read a lot of science fiction when I was 12 to 14. I ran across some scary stuff, such as A.E. van Vogt, who wrote a story called Discord in Scarlet in 1939. It was about a creature that planted eggs in people, and was obviously the basis of the movie, Alien.
Van Vogt said he based the story on a wasp that laid eggs in a certain spider. The wasp paralyzed the spider and as the eggs hatched they ate the spider.
I can't imagine The Walking Dead being popular in the '50s and '60s, not with Leave It To Beaver and Andy Griffith and Gilligan's Island. Or silly joke monsters like The Munsters or The Addams Family.
When did the TV portrays go bad? I'm not sure. I know that Breaking Bad was a huge hit but portrayals on TV had broke bad before then. Seventies, no. Eighties, no. X-Files, yes. It had parasite monsters, too. Kolchak in the '70s, which was an influence on The X-Files, but it was more goofy-fun that anything else.
This slide appears to have been a slow one, and I'm not quite sure when it started. But I do know that if we still have a good economy, and no parasite Third Worlders, and no bloated parasite government, the country wouldn't be breaking down, and we wouldn't have any of these TV programs.