One of the best episodes in my opinion, featured a trip to I believe it was the Atlanta branch of the CDC. It briefly touched on the big picture, but proved nothing more than a tease for everybody ending up running around in the woods.
Then, a year or two ago, I think some promotional materials showed a glimpse of I think Washington DC. So I expected that something relevant was going to happen, but alas was misled again.
They can't and won't do anything on a grander scale.
Yes, I enjoyed that episode also.
I think the desire or need by the writers to explain more of why it happened disappeared when Frank Darabont, the first show-runner and really the guy who brought the story to TV was fired between seasons 1 & 2.
The CDC story never occurred in the graphic novel– Darabont thought it was an important part of his view of "The Walking Dead" story. Robert Kirkman, who was behind the graphic novel never had any interest in explaining the "why" behind the outbreak.
For Kirkman it was all about people vs. people. But that got old after a while.
More on the "why" and trying to find a cure would have broken up the monotony of "all villain's all the time".