Was gonna come in here and say that humans are naturally selfish to some extent but, despite that, maybe people had the virus but just didn't know, weren't aware or weren't diagnosed with it? So didn't know to self-isolate? Bound to be some cases like that, no? Like they might have it but not to a serious, debilitating level so they'll just carry on as normal without knowing they've contracted it. And before you know it they've passed it on...