There is no single reason to explain all the surges. In some cases, more testing has revealed more cases. In others, local outbreaks are big enough to push statewide tallies higher. But experts think at least some are due to lifting stay-at-home orders, school and business closures, and other restrictions put in place during the spring to stem the virus’s spread.

The virus is also gradually fanning out.

“It is a disaster that spreads,” said Dr. Jay Butler, who oversees coronavirus response work at the U.S. Centers for Disease Control and Prevention. “It’s not like there’s an entire continental seismic shift and everyone feels the shaking all at once.”

The virus first landed on the U.S. coasts, carried by international travelers infected abroad. For months, the epicenter was in northeastern states. More recently, the biggest increases have been in the South and the West.