The Delta variant, which was first detected in India, now accounts for more than 6% of all infections in the United States, according to the Centers for Disease Control and Prevention. And this highly transmissible variant may be responsible for more than 18% of cases in some Western states. The variant, also known as B.1.617.2, is spreading rapidly in the U.K., and has quickly become the dominant strain there, responsible for more than 60% of infections and causing surges in some parts of England. "We cannot let that happen in the United States," says Dr. Anthony Fauci.