Florida, Texas and California account for 27.4% of the 328 million people living in the US, according to the latest US Census Bureau estimates.
And while some politicians say the higher number of infections is due to increased testing, that is not the case, said Michael Osterholm, director of the Center for Infectious Disease Research & Policy at the University of Minnesota.
As new cases and hospitalizations skyrocket, Texas Gov. Greg Abbott urged people to stay home.
"Because the spread is so rampant right now, there's never a reason for you to have to leave your home," Abbott told CNN affiliate KBTX. "Unless you do need to go out, the safest place for you is at your home."
Officials encouraged mask wearing and social distancing in places like bars that are often overcrowded. Further state actions could be announced if the virus continues to spread at this rate, Abbott said.
In the nation's most populous state, Gov. Gavin Newsom pleaded with Californians to think of others by wearing masks, keeping a safe distance and washing their hands regularly.
He told residents to "love thy neighbors, like yourself, please" and urged younger people to be especially cautious.
"Be careful about Mom and Dad, and careful about your mother-in-law, your father-in-law, your grandparents," he said.
Florida Gov. Ron DeSantis has attributed the rise to more testing but others say community transmission is playing a key role as the state reopens. Miami Mayor Francis Suarez said he'll ask the city commission to implement a civil fine of up to $250 for those not wearing a mask in public.
Miami implemented an order requiring masks or face coverings in public this week.
"Going out in public without a mask is like driving drunk," said Dr. Jonathan Reiner, a cardiologist and professor of medicine at George Washington University. "If you don't get hurt. You might kill somebody else."