“Let’s see, viruses. Viruses might be good. They seem vulnerable to viruses. And Viruses are tricky, always mutating, and forming new strains whenever a vaccine is developed. And perhaps this first virus can be one that compromises the immune system of these creatures. Perhaps a human immunodeficiency virus, making them vulnerable to all sorts of other diseases and infections that might come along, and maybe it could be spread sexually, making them a little reluctant to engage in the act of reproduction.”
George Carlin