I've always like the idea of someone releasing a nanovirus that slowly turns all the world's reproductive-aged women into barnyard animal girls (cowgirls, piggirls, etc) while simultaneously wiping out the female half of all barnyard animal species, forcing men to seriously consider using women en masse to replace all of those missing female animals in farms (and in some cases, even in the wild) to keep those species from going extinct and to keep the meat, dairy, livestock and textile industries from being permanently destroyed.