《打预防针，我不怕》

今天是朵朵打预防针的日子，朵朵最讨厌打针啦。医生说朵朵的身体很健康，朵朵很高兴，因为身体健康就不用打针啦。但医生却说，预防针就是要在身体健康的时候才打的。这是怎么回事呢？为什么预防针要在身体健康的时候打呢？预防针为什么能预防疾病呢？
Today is Duoduo's day of vaccination. Duoduo hates injections the most. The doctor said Duoduo is in good health, and Duoduo is very happy because she doesn't need to be injected. But the doctor said that the vaccine should be given only when the body is healthy. What's going on? Why should people are vaccinated when they are healthy? Why can vaccine prevent disease?