Why is COVID surging? A doctor explains what to know and how to stay safe.

It's not just President Biden who recently tested positive for COVID — cases of the virus are spiking across the country.

Nearly 40 states are reporting high COVID activity levels, according to data from the Centers for Disease Control and Prevention, and emergency room visits are at their highest for the virus since February.

Why the seemingly sudden summer surge? There are a couple of factors at play, Dr. Céline Gounder, CBS News medical contributor and editor-at-large for public health at KFF Health News, told "CBS Mornings" Wednesday .

"One, the virus continues to evolve to stay ahead of our immune systems. That's what we can talk about when we're talking about variants," she said. "Two, your immunity to infection only lasts about three months. Your immunity to severe disease, hospitalization and death, that lasts much longer, which is why people are not getting sick the way they were early in the pandemic. But it is to be expected that every few months, maybe twice a year or so, we'll see a big wave of COVID across the country."

More reasons for increased numbers? People are traveling and spending more time indoors and not masking as much, Gounder added .

These reasons add to the concerning potential for a COVID-19 outbreak to spread within the tightly confined 2024 Summer Olympics , as thousands of athletes and spectators from around the world have descended on Paris.

Current guidelines, however, can help keep people safe.

"You should be staying away from others for at least 24 hours, at least until your fever resolves without the help of a medication like Tylenol, and your symptoms are improving," Gounder said. But you should, as much as possible, take additional measures "for at least five more days, which is when you're most infectious, most likely to transmit to other people."

Options for this include:

• Continuing to isolate
• Wearing a mask when you're around other people
• Opening windows
• Using HEPA air filtration units to reduce the risk of transmission

Gounder also urges people to use "common sense" when it comes to COVID testing and precautions.

"If you're feeling sick, probably should get tested. When you're feeling sick, probably shouldn't be around other people to the degree that you can avoid that," she said. And masks, "contrary to some opinions, do work to protect you if you're wearing a N95 or KN95 mask, and they also work to protect other people if you're infected."

• Is masking coming back? As new COVID variants emerge, here's what experts say.
• Long COVID has affected millions. Here's what scientists now know.