If smallpox gets out in a big way, it won't matter if they sic it on the Americans or the Germans first. It's incredibly contagious. Them that's vaccinated will survive, half of them that ain't will die, and the other half will be living their lives looking like someone threw acid in their face.
(My guess is, both we and the Krauts will be able to vaccinate everyone, and the third world will die. Oh well. Shoulda supported us in the Security Council)
Both George Washington and Abraham Lincoln survived smallpox, Lincoln while he was President.