Replacing copper by steel made humanity less resilient to bacteria
One of the first metals that were used by humanity to produce tools was copper, most likely in the form of bronze, hence the bronze age. It turns out that copper has antimicrobial properties1 that helped storing produce for longer and in a safer way.
After the war (which one?), more modern materials were favored, such as steel and plastics2, but they don't have any antimicrobial properties. Therefore, humanity became less resilient to the spread of microorganisms at the same time copper's use enter into a decline.
Antimicrobial coatings are gaining momentum, and are based on a lot of the knowledge generated by people researching copper's properties.