At a Maryland country fair in 2017, farmers reported feverish hogs with inflamed eyes and running snouts. While farmers worried about the pigs, the department of health was concerned about a group of sick fairgoers. Soon, 40 of these attendees would be diagnosed with swine flu. How can pathogens from one species infect another, and what makes this jump so dangerous?

在2017年马里兰州的一个乡村集市上，农民报告说，有发烧的猪，眼睛发炎，流鼻涕。当农民担心猪的时候，卫生部门担心的是一群生病的集市观众。很快，这些参会者中就有40人被诊断出患有猪流感。一个物种的病原体是如何感染另一个物种的，是什么让这种跳跃如此危险？