Doctors administer vaccines before a person has been ex posed to such diseases as poliomyelitis or smallpox. Vaccines are usually injected, but sometimes are given orally.
在人接触某种疾病，如脊髓灰质炎或天花之前，医生接种疫苗。疫苗通常注射，但有时口服。
Doctors administer vaccines before a person has been ex posed to such diseases as poliomyelitis or smallpox. Vaccines are usually injected, but sometimes are given orally.
在人接触某种疾病，如脊髓灰质炎或天花之前，医生接种疫苗。疫苗通常注射，但有时口服。
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