致病原因

例句：

1. 1. The doctor found that the patient's illness was caused by a virus.
   医生发现病人的疾病是由病毒引起的。
2. 2. Epidemics are often traced back to specific causes, such as a new strain of bacteria.
   流行病通常追溯到特定的原因，比如一种新的细菌株。
3. 3. Researchers have identified several factors that contribute to the onset of Alzheimer's disease.
   研究人员已经确定了几个导致阿尔茨海默病发作的因素。