After a minor late-spring lull, the number of confirmed coronavirus cases in the U.S. is once again on the rise. States like Arizona, Florida and Texas are seeing some of their highest numbers to date, and as the nation hurtles further into summer, the surge shows few signs of stopping.

And yet the virus appears to be killing fewer of the people it infects. In April and May, COVID-19 led to as many as 3,000 deaths per day, and claimed the lives of roughly 7% to 8% of infected Americans. The number of daily deaths is now closer to 600, and the death rate is less than 5%.

In general, experts see three broad reasons for the downward trend in the rate of coronavirus deaths: testing, treatment and a shift in whom the virus is infecting. The relative contribution of these factors is not yet clear. And because death reports can lag diagnoses by weeks, the current rise in cases could still portend increases in mortality in the days to come.

Testing on the rise

Since mid-March, when the coronavirus was declared a national emergency, diagnostic testing has risen significantly. More than 600,000 tests are administered each day in the U.S., up from about 100,000 per day in early spring. Although the nation is still falling short of the millions of daily tests that experts have called for, the increased testing has identified many more infected individuals with mild or no symptoms, driving down the overall proportion of patients who die from COVID-19, said Caitlin Rivers, a senior scholar at the Johns Hopkins Center for Health Security.

And with more tests available, infections are often identified earlier, “which allows us to intervene earlier,” said Saskia Popescu, a hospital epidemiologist and infectious disease expert in Arizona. Many treatments seem to work best when given well before people are at death’s door.

As the weeks have worn on, doctors and nurses have also gained a better handle on how to treat the virus. In several states, emergency departments are no longer overflowing; between April and June, nationwide hospitalizations dropped to less than 30,000 from nearly 60,000, according to the COVID Tracking Project. That may have eased the strain on exhausted employees and limited medical supply chains, including those that keep lifesaving equipment like ventilators in stock, said Dr. Taison Bell, a physician specializing in infectious disease and pulmonary and critical care at the University of Virginia. Under less pressure, hospitals are now “better able to take care of critically ill patients,” he said.

More and better treatments

Health care workers have also become more knowledgeable about promising treatments and palliative care options to combat the coronavirus and its effects. For instance, prone positioning, in which patients are flipped onto their stomachs, can ease respiratory distress by opening up the lungs. Critically ill individuals are also now known to be vulnerable to excessive blood clotting and may benefit from blood thinners. And the steroid dexamethasone appears to reduce deaths among patients with severe COVID-19, although the data demonstrating this emerged only recently. (Another drug, an antiviral called remdesivir, seems to speed recovery but does not appear to have notable effects on mortality.)

“Before, it felt like we were stumbling in the dark,” Bell said. “It feels a little bit better now.”

New patient population

A shifting patient population is probably also altering the disease’s dynamics. Coronavirus-related hospitalizations increase with age, and elderly individuals remain some of those hardest hit by the coronavirus; patients over 65 account for 8 out of 10 deaths from COVID-19, according to the CDC. But younger people now make up a growing proportion of cases, and they are less likely to die from the disease. In Arizona, people ages 20-44 now account for nearly half of all cases. In Florida, which just recorded more than 10,000 new cases in a single day, the median age of residents testing positive has dropped to 35 from 65. And in Texas, more than half of those testing positive are under 50.

Numerous states recently began reopening their economies, which might be driving some of the youthful bias, said Natalie Dean, an infectious disease epidemiologist in Florida, where new cases are hitting record highs. People in their 20s and 30s have returned to bars and beaches; working-age employees have resumed jobs that can’t be done from home.

“We know that’s high-risk,” Dean said. “We’re hearing a lot of reports of clusters being linked to these places” as they open back up.

At the same time, elderly individuals, as well as those with underlying health conditions thought to exacerbate COVID-19, may be warier of exposure, said C. Brandon Ogbunu, a computational biologist and disease ecologist at Yale University. “Early on, this disease ripped through older populations with such aggression,” he said. “It’s possible that’s where the message was felt the most strongly.”

Looking ahead

Experts can’t be sure, but behaviors like mask wearing, physical distancing and hygiene may also be reducing the dose of coronavirus that people encounter in the population at large, Dean said. The amount of virus that individuals carry may influence the severity of their symptoms. But so far, there is no evidence that this dynamic is contributing to the lower mortality rate in the United States.

There is also no indication that the death rate is lower because the coronavirus itself has become less deadly, Ogbunu said.