二十世纪中叶,天花、脊髓灰质炎、疟疾、结核病和婴幼儿死亡,是世界上低收入国家面临的主要健康问题。在21世纪末,情况发生了巨大的变化。天花已被消除。在不久的将来,脊髓灰质炎也将被消灭。婴幼儿死亡率显著下降,针对一些幼年类病进行的免疫每年 By the middle of the twentieth century, smallpox, poliomyelitis, malaria, tuberculosis and the deaths of infants and young children are the major health problems facing low-income countries in the world. At the end of the 21st century, the situation has changed dramatically. Smallpox has been eliminated. Poliomyelitis will also be eliminated in the near future. Infant mortality rates have dropped significantly, with immunizations against some juvenile diseases each year