Final answer:
The 'silent killer' described in the riddle is the unseen threat of submarine warfare during World War I, specifically relating to the sinking of the RMS Lusitania in 1915, which led to civilian deaths and was a pivotal event leading up to U.S. involvement in the war.
Step-by-step explanation:
The riddle presented seems to be describing a historic event, and the reference to 'a silent killer sleeps aboard on Europe's wooden ships' might initially sound like something out of Biology with a suggestion of a pathogen or disease. However, the historical context provided by phrases such as 'American lives are in my grip' and mentions of German submarines suggests we are talking about a maritime disaster during wartime, in which unsuspecting victims onboard ships may suffer due to hidden dangers, just like silent diseases, but here the cause is human-engineered warfare.
One significant historical event that fits this description is the sinking of the RMS Lusitania in 1915 by a German submarine. The occurrence was instrumental in causing public outrage and eventual U.S. involvement in World War I. The 'silent killer' in this context is not a disease but the unseen threat of submarine warfare that targeted merchant and passenger ships, thereby having a huge impact or being a 'big deal' as the riddle implies.