When you are tired, run down, on not eating properly, you are more vulnerable to infection.

【正确答案】 C

【答案解析】解析：题干中vulnerable to“容易受……的伤害”，resistant to“有抵抗力的”，safe from “避免”，open to“为……开放”；desirous of“渴求”，选项C最接近题干意思。