Where medical science had advanced so much, they were able to totally eradicate all known diseases, infections, viruses etc to the point where humans no longer had, or needed, an immune system. Of course nature always bites back and sure enough one small virus appears and virtually wipes out humanity. But just as all looks lost, a new generation develops an immune system but it takes many years to recover and total destruction of society to boot. Recession and depression aren't in it. Innit? Was that a film? Or did I have a nightmare last night? :o)