There are several reasons for this. For starters, the new mini-ball bullets would fracture bones and while in today's time, a fractured bone would just require intensive surgery, they lacked the technology for that kind of surgery and these wounds would be treated by the full amputation of the entire limb. This strategy made it more likely for patients to contract infections from their wounds and the problem would only be amplified by how doctors in this time did not believe in cleanliness as they saw that the dirtier they were, the more experience that was upon them. These diseases weren't just from wounds either, armies also did not understand how infectious diseases were spread and because of this, diseases like dysentary, typhoid, and typhus would spread like wildfire across armies.