Since the end of 2019, an outbreak of COVID-19 of unknown origin has been attacking human beings and sweeping the world. As of May 17, more than 4 million people worldwide have been infected with the virus, and more than 300,000 people have lost their lives.
The US learned about the novel coronavirus and acted almost at the same time as China. However, four months later, the US still is the center of the epidemic. What is the cause of this?
(Produced by Chen Lidan, Wang Xiangyu, Yan Yiqiao and Jiang Yunan)