As the US coronavirus death toll reached the grim milestone of more than 100,000 deaths this week, world health experts are concerned about new hot spots and the Centers for Disease Control and Prevention is projecting 123,000 coronavirus deaths by late June.

More than 1.7 million people in the US have tested positive for coronavirus and over 102,500 have died, according to data from Johns Hopkins University. The US Centers for Disease Control and Prevention projects that by June 20 the death toll will reach about 123,000.

The projection is based on an ensemble forecast of more than a dozen individual forecasts from outside institutions and researchers. The projections show a possible range of 115,400 to 134,800 deaths before the end of next month.

"This week's national ensemble forecast indicates that the rate of increase in cumulative COVID-19 deaths is continuing to decline. Nevertheless, total COVID-19 deaths are likely to exceed 115,000 by June 20," the CDC says on its website.

Meanwhile, as Americans continue reopening their businesses and returning to public spaces after lockdowns to stop the virus's spread, the World Health Organization is still keeping an eye on hot spots in other parts of the world

Among them are Russia, Africa, the Americas, some countries in South Asia and a few countries in Europe, said Maria Van Kerkhove, technical lead for the WHO's coronavirus response.

"These are areas that we are concerned about because, as we know, as you know, when this virus has an opportunity to really take hold it can grow very, very quickly," Van Kerkhove said. "Any one of these could really take hold and take off very, very quickly and we have seen how this virus affects vulnerable populations, and that's a big worry of ours."

Live updates: Coronavirus in the US

Behavior that can make a difference

With neither a vaccine nor approved treatment available for Covid-19, experts say the best defense against a resurgence is public behavior.

Socializing outdoors, maintaining distance from others and wearing face coverings have been highlighted as best practices for reducing coronavirus transmission.

Face masks worn at home can help to stop the spread of coronavirus among people who live together, according to a study published Thursday in BMJ Global Health. Researchers in China studied 124 families with at least one coronavirus patient.

The study found that opening windows, keeping more than about 3 feet apart and disinfecting shared surfaces also seemed to lower the risk of passing the virus on to family members, even in crowded homes.

If a person wore a face mask before they showed signs that they were sick, it was 79% effective at reducing transmission, according to the study.

The masks proved only to be effective in the home before a member of the household showed symptoms, the study said.

But it is still an important precaution in all settings, because many of the people carrying the virus don't know they are infected, said Erin Bromage, associate professor of biology at the University of Massachusetts Dartmouth.

"A standard mask, the ones that we've been making, cut things down by 50%. I wear it to protect you, you wear it to protect me," Bromage said.

Behavioral changes have already proven successful in bringing cases down in China, New Zealand and Australia, said Dr. William Haseltine, president of the think tank ACCESS Health International.

Understanding the scale of coronavirus in the US

Tools in the fight called into question

Long-emphasized precautions continue to be crucial to reducing transmission, health experts say, but measurements the US has relied upon from the start of the pandemic are now being called into question.

Maintaining 6 feet of distance from another person has been the standard in social distancing to reduce airborne transmission, but three experts have questioned if that is enough.

Evidence shows that aerosols can accumulate and remain infectious indoors for hours, so particles still in the air could be easily inhaled into the lungs -- even if people are standing 6 feet apart, according to the commentary published in the journal Science.

Meantime, both diagnostic and antibody tests have shown limitations this week.

Many people admitted to the hospital with severe illness need multiple coronavirus diagnostic tests, often advancing to tests from deeper in the respiratory system, before the test comes back positive.

Coronavirus antibody tests may be wrong half the time, the US Centers for Disease Control and Prevention said in new guidance posted on its website this week. Antibody testing looks for evidence of immune response to determine if a person had been infected in the past.

The tests are not accurate enough to make policy decisions, the CDC said.

The state of coronavirus

States have been taking varied approaches to reopening and also contending with different rates of infection.

As of Friday afternoon, 15 states were showing an upward trend in average daily cases -- a rise of at least 10% -- over the previous seven days, according to an analysis of Johns Hopkins University data.

Twenty-five states' average daily cases dropped more than 10%, while the rest were level or near level, the data showed.

California on Thursday reported its largest single day increase in confirmed cases since the pandemic began. Georgia also saw a spike in cases this week, but Gov. Brian Kemp attributed the growth to a recently submitted backlog of test results.

"I can assure you that (Georgia Department of Public Health Commissioner Dr. Kathleen Toomey) and I and our whole teams continue to watch that data," Kemp said.

Attractions geared toward drawing large crowds are making their way back into the US.

The Las Vegas strip will open back up June 4 -- though not without precautions in place.

"We're welcoming visitors back, but we're going to take every precaution possible," Nevada Gov. Steve Sisolak said in an online briefing. "We're encouraging visitors to come and enjoy themselves and have a good time."

In Florida, Miami-Dade County Mayor Carlos Gimenez announced that beaches and hotels may open on Monday and Walt Disney World on Wednesday proposed reopening Magic Kingdom and Animal Kingdom on July 11, and EPCOT and Hollywood Studios on July 15.

Los Angeles County is expanding its reopening and residents will be able to dine in restaurants and shop in retail stores once again, including salons and barbershops. They must meet certain safety guidelines.