Soldiers returning from World War I brought back disease to the United States, most notably causing the deadly Spanish Flu epidemic in 1918. An H1N1 virus, the Spanish Flu resulted in upwards of 50 million deaths worldwide, around 700,000 of which were US citizens. The flu's deadly impact and its global spread existed as a negative byproduct of the growing interconnectedness of world affairs. With more and more individuals interacting with each other around the globe (be it due to war, trade, or travel), the contraction and spread of diseases became a much larger problem than in earlier periods of isolation.