When David Bloom, 39, went to Iraq in 2003 to cover the war for NBC News, his wife, Melanie, naturally feared for his safety. Would a bullet or a bomb claim him? A land mine? An ambush? Instead it was a blood clot lodged in his lungs that ended his life. Ms. Bloom subsequently learned that her husband carried a genetic abnormality, factor V Leiden, that greatly increased his risk for developing blood clots. Mr. Bloom had three other risk factors for clots: a long plane ride to Iraq, erratic eating habits that could have caused dehydration, and cramped sleeping...