Infection:

Infection of a wound was a major cause of death
Bullet and shell fragments carried materials such as mud and soil into the body which often became infected
Many soldiers recovered from initial injuries but died from infections
The infection that caused the highest number of deaths was gas gangrene (carried by bacteria living in soil and develops rapidly)
Wounds infected by this bacteria swelled up with gas
Before the development of antibiotics, doctors could do nothing to fight this infection