The 'last' weapon isn't as crucial as the most important new weapon, which was the Gatling Gun---The first machine gun.
Sanitation methods progressed little during the war. For example - I saw this at a civil war re-enactment battle - They would wrap a soldier's wound with linen bandages, let them dry, and then re-use the bandages on the next fellow. The problem was, Louis Pasteur was just then - about 1865 to 1880 -developing the 'germ' theory of where disease came from; and was starting to encourage doctors to clean their instruments from one patient to the next. If people did not think that germs caused disease, there was no reason to practice sanitation.