But there are still many diseases for which vaccines do not yet exist. Moreover, strategies that have previously led to the successful development of vaccines are unlikely to work against more complex bacteria or viruses, such as HIV, which have evolved multiple mechanisms to evade the immune system.
但仍有许多疾病我们没有疫苗可用。此外，早前带来成功疫苗开发的策略不再适用于更加复杂的细菌和病毒，比如可通过多种机制削弱免疫系统的艾滋病毒。