In this uphill battle against Covid-19, letting your foot off the gas too soon can send you backward.
That's what places like New York City are discovering after months of steady progress in fighting the pandemic.
And this is a very bad time to be going backward, as Americans face a trio of major challenges this fall: more indoor gatherings, a potential coronavirus-and-flu "twin-demic," and trying to get or keep students back in school safely.
Nationwide, 21 states reported more new cases this past week compared to the previous week, according to data from Johns Hopkins University.
Only 10 states are showing downward trends in new cases -- Arizona, Florida, Georgia, Tennessee, Virginia, Connecticut, Vermont, New Hampshire, Delaware and Maryland -- and 19 states are holding steady.
But in many states, "it's not only that the number of infections keeps on going up. It's also that the test positivity rates are trending in the wrong direction," emergency medicine physician Dr. Leana Wen said Monday.
A test positivity rate is the percentage of test results that turn out to be positive. The World Health Organization has recommended test positivity rates stay at or below 5% for at least 14 days before businesses reopen.
"We're seeing more than a dozen states with a test positivity is over 10%. And there are two states -- Idaho and South Dakota -- where the test positivity is over 20%," Wen said.
"That means that not only do we have increasing infections in these states, we also don't have nearly enough testing."
Cases 'grow at an alarming rate' in parts of NYC
For the first time since early June, New York state reported more than 1,000 new cases Saturday -- prompting Gov. Andrew Cuomo to warn that this virus "remains a force to be reckoned with throughout the country."
"It's vital that New Yorkers continue to practice the basic behaviors that drive our ability to fight Covid-19 as we move into the fall and flu season," Cuomo said. "Wearing masks, socially distancing and washing hands make a critical difference."
In New York City, "COVID-19 cases continue to grow at an alarming rate in eight neighborhoods in the city, outpacing the citywide average by 3.3 times over the past 14 days," the city health department said Sunday.
New York state was once the US epicenter of this pandemic, but the rates of new cases, hospitalizations and deaths have plummeted in recent months.
New York state boasted a test positivity rate of less than 1% for more than a month. But that streak has been broken, with the test positivity rate hovering around 1.02% in recent days.
"We cannot drop our guard," Cuomo warned.
That's especially true as public schools in New York City reopen to students this week. But officials could decide to close schools, limit gatherings and issue fines for not wearing masks.
New York City has reported more than 237,000 of the state's 455,626 total Covid-19 cases. But with a seven-day average of 224 new cases a day, the city is still nowhere near its April peak, when more than 5,000 new cases were reported every day.
Nationwide, more than 7.1 million people have been infected with coronavirus, and more than 204,000 have died, according to Johns Hopkins data.
And health experts warn the US could see an explosion of new infections this fall and winter if people get more lax about precautions and spend more time indoors.
Authorities crack down on large gatherings
Now authorities are stepping up efforts to end gatherings where the virus can easily spread.
Sheriff's deputies broke up a wedding of about 300 people Friday evening in Queens, New York City Mayor Bill de Blasio's office said.
Both the owner and manager of the venue face multiple misdemeanor charges.
In Maryland, a man was sentenced to a year in prison Friday after holding two large parties in late March in violation of the state's social distancing rules, which banned gatherings of more than 10 people, Gov. Larry Hogan's office said.
The man started arguing after he was told to disband his first party of about 50 people, the state attorney's office said. Five days later, he held a second party of more than 50 people that he refused to end, saying they had a right to congregate.