One of the factors that affected the lives of the civilian population was the fact that many cities were attacked and demolished. An example of this is the Battle of Verdun. Verdun is a key city in France, which is the gate for supplies to reach soldiers out on the front. During this battle, over 700,000 people died altogether. Some of them were soldiers, some of them were civilians. The Axis also sieged the city and turned it into a big area of rubble. This no doubt affected the lives of the people who were living in or near Verdun. This played a big factor because many other cities were also destroyed by Axis and Allies alike, which really affected the lives of civilians.
Another factor is the unrestricted submarine attacks against sea trade. The German submarines were sinking every ship that was traveling from The Americas to Europe, which means they were sinking the ships of neutral countries, and cutting off the provision supply to Europe.
This was what eventually caused U.S. to join the Allies, but it also resulted in hunger throughout all Europe. Germany didn't have enough food, and nor did Britain. And since it was war time, most of the food was for the men fighting on the front, which left very little for civilians to eat. Many civilians died of hunger because of this. I think this is the most important factor, since it affected the biggest number of people and countries.
Yet another factor is the diseases that spread throughout the battlefield. An example is the trenches and "no man's land". Many soldiers were dying in No man's land, which meant they can't be retrieved, so they were just left there to rot. This resulted in many diseases spreading not only to the...