Final answer:
During World War I, the Spanish flu spread among American soldiers serving overseas and became a deadly pandemic. The movement of soldiers across borders facilitated the spread of the disease, and the lack of preparedness and knowledge about the flu contributed to its severity.
Step-by-step explanation:
The question relates to the spread of a deadly disease among American soldiers during World War I. The soldier mentioned in the question refers to the American soldiers serving overseas during the war. The disease being referred to is the Spanish flu, which was a pandemic that spread globally during 1918-1919. The spread of the disease was facilitated by the movement of soldiers across different countries.
One of the major contributing factors to the spread of the Spanish flu was the movement of soldiers across borders. As soldiers from various countries came in contact with each other during the war, the disease was transmitted. The crowded and unsanitary conditions in military camps and trenches also facilitated the spread of the virus. Additionally, the soldiers returning home after the war further spread the disease to civilian populations.
The Spanish flu became more deadly due to the lack of preparedness and knowledge about the disease at the time. Medical professionals had never encountered the strain of influenza before, leading to a lack of effective treatments and preventive measures. The virus mutated into a more severe strain, causing widespread illness and death.