Frontline Soldiers in World War One:
The British lost 1.53% of their population in military service during the Great War, the Germans lost 3.23% and the French 3.7%1.
Fatalities in the BEF (British Expeditionary Force) were on average 1 in 7 for officers and 1 in 8 for other ranks, and on the Western Front 10% of these fatalities were caused by something other than enemy action (in East Africa over 70% were non-battle fatalities). In the last month of the war alone, over 2,500 british soldiers died from Spanish Flu.
Furthermore, the British suffered over 6 million non-fatal casualties, some 65% of which were classified as non-battle injuries across all theatres, but the British only reported injuries which prevented a soldier from reporting for duty, regardless of how they were caused. Thus if you were going to die then it was likely to be the enemy who were responsible, but injury was more likely to come from a different source. Also many soldiers would suffer multiple wounds so it is very difficult to estimate the chances of a British soldier being wounded during a particular period of time.
The issue is further clouded by the definition of a front line soldier. Before the war it was easy to categorise the infantry, artillery and cavalry as front line (a habit which persists in the Army to this day), but come the middle of the war an artilleryman could find himself posted from the field artillery, a front line unit, to garrison artillery, which sat much further back. Also infantry units were rotated in and out of the front line on a regular basis and so, by sheer caprice, some found themselves in contact with the enemy more often than others. The reach of artillery also brought soldiers in traditionally safer occupations (e.g. drivers or medics) in to the midst of battle.
The census data from 1911 to 1921 shows that 14% of men who would have been of fighting age at some point during the war died during that period. The vast majority of these are for non-war related reasons and for soldiers from the most impoverished sectors of society, life on the Western front was healthier and less dangerous than civilian life at home.
*This gives you some idea why the French did everything they could to avoid war in 1939/40, including their failure to go on the offensive in any meaningful way.