How diseases spread,

病情如何蔓延
how they can be treated or cured,

应该怎样治疗或治愈疾病
when they kill somebody.

它们什么时候会置人于死地
The fact of the matter is,

事实上
demon theory doesn't do a very good job of explaining disease,

恶魔理论并不能很好地解释疾病的问题
while germ theory does do a good job.

而微生物理论却能将问题分析得很透彻
It's the better explanation.

相较之下微生物理论是更好的解释
So we're justified in believing in germs,

所以我们相信微生物理论合理
but not demons.

而恶魔理论则不合理
It's a matter of inference,

这是逻辑推论的结果
not just to any old explanation,

不是随便什么陈旧的解释
but inference to the best explanation.

而是最佳解释的推论结果