Billions of free roaming bioreactors fully capable of catching and spreading the disease, there to provide the medium for countless mutations and variants, with ostensibly less severe symptoms and outcomes being the carrot dangled in front of the population to get them to all join in.
If that didn't work, Mandates.
This situation has been foreseen, even in these pages, and by people who have no claim to be medical experts other than a bit of knowledge and a lot of common sense.