Now, as the country sweats through the thick of what is likely to be another sweltering summer, another invisible threat looms.

This weekend, as a heat wave bakes huge swaths of the country under triple-digit heat indexes, some fear that the collision of Covid-19 and extreme heat could be a dangerous combination.

Extreme heat can threaten anyone, but many of the same groups who are at greatest risk of serious illness from the coronavirus are also the most vulnerable groups to heat exposure.

With indoor gatherings known to facilitate the spread of Covid-19, cities and relief organizations are adjusting how they keep people safe in this new normal.

And with millions across the country out of work and the virus forcing vulnerable people to stay in their homes, experts say the pandemic is compounding the heat risk for those who are already struggling.

"For staying safe from coronavirus, the message is that you're safer at home," said Dave Hondula, a researcher at Arizona State University's Urban Climate Research Center who studies extreme heat and health. "That is true from a coronavirus perspective, but if you don't have sufficient cooling, or are not able to control the temperature of your home, you may not be."

How heat kills

Though it tends to get less attention than other weather events, extreme heat can and, tragically, does kill.

It is one of the most deadly types of weather-related events in the US, killing an average of 702 people each year, according to Paul Schramm, the climate science team lead at the Centers for Disease Control. That's more than are killed in hurricanes, floods and tornadoes in most years, he says.

When high temperatures combine with stifling humidity -- as they are expected to do this weekend across the Midwest, South and East -- it can overwhelm a person's ability to sweat and cool down, leading to spikes in body temperature that can damage the brain and other organs.

And when temperatures stay high with little cool down overnight, the heat risk can rise even further.

The threat is greatest for the elderly, young children, people who are overweight, and those who work outside or exercise during the hottest part of the day, according to the CDC.

Experts also point to social isolation as a concern when it comes to extreme heat. Schramm recommends that people check in on at-risk family members or neighbors at least twice a day to look out for signs of heat exhaustion.

Cities are changing how they keep residents cool

We know that staying home and away from others is the best way to prevent the spread of Covid-19.

But if people are stuck inside without a means to keep cool, they could be trading one danger for another.

The CDC has issued guidance for how to safely open cooling centers, which many cities are still offering.

However, many of the informal cooling centers people normally use to get a break from the heat -- malls, libraries, pools and grocery stores -- are either closed i