Sleep well....
Or similarly, a fragment as contgagious as the flu - but incapable of causing serious illness, and combined with genes which give the "infected" permanent protection against the one hundred most common types of cancer.
Yeah, and maybe the survivors will start to have dreams about an old granny lady from Hemingford Home, Nebraska. And then they'll all either move to Boulder or Las Vegas. Then maybe some deranged pyromaniac will find a nuclear weapon out in the desert and bring it back to Vegas and the hand of God will ignite it.