Much has been said about the miracle that is nano-vaccine, and the great conditions provided by planned communities like the NEST system. However, the truth is these things were not made simply to acknowledge Utopian dreams. We don't like being killed by hurricanes or starving when food deliveries are cut off and worsening environmental situations in the thirties and forties were threatening just that. To this day, the weather can be as much of a threat as the reanimates.
An odd little fact for you: King Gillette,- the man who literally and figuratively changed the face of capitalism by popularizing the disposable razor, was a socialist. In 1894 he wrote a book called "The Human Drift" about how to create a workers' utopia. It proposed moving the entire population of the United States to a giant city in New York state that derives all the electricity it needs from hydro-electric dams placed across Niagara Falls. In turn, economies of scale from factories running off this abundant power (derived from Tesla's recently developed AC current generators no less) would allow everyone to be clothed and housed.
Of course, relocating entire populations and building super cities proved to be far too difficult. Land prices remained cheap, food in fairly abundant supply, and existing infrastructure means it was far quicker to expand existing cities and towns.
However oil shortages and changing climate in the late twenties and early thirties changed that. Sea levels and storms foxed people away from coasts, desertification started immigration to more habitable climates, and crop failures drove up food prices and delivery charges - making life away from existing hubs difficult. As present as mild winters are - a good cold snap is needed to kill the vermin that carry disease - warm climate and refugee camps began to yield epidemics, and threatened pandemics.
Not every place got warmer - it would be more proper to call the process global shifting than warming really - some places in Europe got very cold, and if they didn't - temperature deferential made for more powerful storms. Adding to the chaos - when everything shut down during The Event - cities stopped generating micro-climates. You might hear about the small town that gets wiped off the map by a tornado - but they don't hit downtown Chicago, because the heat and altered wind patterns around buildings break them up.
The silver lining to this might be that with industry stopped, oil use at a minimum, and populations reduced the Earth might heal all these problems. But its not not going to be overnight, or even within one or two lifetimes. Storms will be bad, drought will be as likely to kill as reanimates, and home grown mosquitoes far more dangerous than invasive aliens. Come to think of it - I don't think citizens can catch Earthly diseases - so we may be wiped out by Lyme disease rather than Laser towers.