一种发生的途径是人类同时感染了H5N1和普通流感。

One way in which this could happen is if a person is simultaneously infected with H5N1 and a common human flu virus.

youdao

问：在中国是否有许多人同时感染了艾滋病毒和其他性传播感染？

Q: Do many people in China have both HIV and other STIs?

youdao

另一种途径是两种病毒同时感染人类密切接触的另一种动物，例如猪。

Another way would be for both types of virus to infect other animals with which humans have close contact, such as pigs.

youdao

You want to give it to children because it's children that are susceptible, and that's where you would like the vaccine to be useful is in children.

你想把疫苗分发给孩子们,因为他们最易受到感染,同时你也希望,疫苗可以对孩子有效

耶鲁公开课 - 生物医学工程探索课程节选

Basically, injecting it into some test subjects, making sure that they didn't get diseased from it and looking at antibody responses to see if it worked and it did.

他们把疫苗接种到测试对象体内,确定接种者不会感染疾病,同时观察抗体的应答反应,了解疫苗是否有效

耶鲁公开课 - 生物医学工程探索课程节选