Death tolls in old wars usually are less than in more recent ones, if only because it is easier to kill a lot more people with one weapon today. Though this is changing in some conflicts (not even wars) such as the ones in Iraq and Afghanistan (though it could be considered a war on the Iraqi/islamofascist side—they’re losing a lot of people, though not in the hundreds of thousands).
That depends. A lot of people starved or died of plagues spread by armies. Direct battle deaths didn't actually exceed "other causes" until armies instituted regular supply systems & stopped living off the land. Also, innoculations & antibiotics helped limit "camp deaths".
The Allied Expeditionary Force probably lost more men to the Spanish Influenza of 1919 than it did in combat in 1918. The US Army in Cuba (1898-9) was decimated by Malaria.