Bird Flu
All reputed scientists* agree that bird flu will definitely be the next Black Plague, and will in all likelihood wipe out 99% of the world's population. To this end, we have devised a plan to evacuate a select number of infection-free individuals to our friendly neighbouring planet, Mars. This carefully-selected team of environmentalists and supermodels will be charged with ensuring the continued existence of mankind through an extensive breeding program. Also, a second spaceship - currently codenamed "The Ark" - will be used to carry out a similar mission, but with animals in place of the environmentalists. Should be... kinky.
Meanwhile, practically everyone left behind on Earth will inevitably contract bird flu, grow wings, and fly too close to the sun and burn up. This is in accordance with the ancient prophecy, and, indeed, can be viewed as natural selection at work (which is why we don't see fit to intervene).