When the coronavirus began sweeping around the globe this spring, people from Seattle to Rome to London canceled weddings and vacations, cut off visits with grandparents and hunkered down in their homes for what they thought would be a brief but essential period of isolation.

But summer did not extinguish the virus. And with fall has come another dangerous, uncontrolled surge of infections that in parts of the world is the worst of the pandemic so far.

The United States surpassed 8 million known cases this past week and reported more than 70,000 new infections Friday, the most in a single day since July. Eighteen states added more new coronavirus infections during the seven-day stretch ending Friday than in any other week of the pandemic.