The Walking Dead2010

[Glenn and Maggie talk to Eugene about after they arrive in Washington D.C]

Maggie Greene:
How long will it be? After you get on that terminal and do what you have to do?

Dr. Eugene Porter:
Depends on a number of factors including density of the infected around target sites worldwide.

Glenn Rhee:
Wait, target sites? Are you talking about missiles?

Dr. Eugene Porter:
That's classified.

Glenn Rhee:
I thought we were over that.

Dr. Eugene Porter:
What if we all live?

Glenn Rhee:
The secrets will matter then?

Dr. Eugene Porter:
They might. Anyway, the speed with which things normalize depend on a number of factors including worldwide weather patterns, which were modeled without the assumption that cars, planes, boats, and trains wouldn't be pouring hydrocarbons into the atmosphere this long. Changed the game quite a bit when it comes to air-transmissible pathogens.