A new study, published online Wednesday, sheds light on why. It finds that the variant grows more rapidly inside people's respiratory tracts and to much higher levels, researchers at the Guangdong Provincial Center for Disease Control and Prevention reported. On average, people infected with the delta variant had about 1,000 times more copies of the virus in their respiratory tracts than those infected with the original strain of the coronavirus, the study reported. In addition, after someone catches the delta variant, the person likely becomes infectious sooner. On average, it took about four days for the delta variant to reach detectable levels inside a person, compared with six days for the original coronavirus variant.