As the new year begins and the depths of winter approach, U.S. infectious disease experts monitoring the "tripledemic" stew of viruses that have been plaguing the country say there's good news — and bad.

The good news is the worst appears to be over from the RSV surge that has been making life miserable for many children and their parents. RSV cases have been falling steadily since the end of November, according to the Centers for Disease Control and Prevention.

At the same time, the flu — which also came roaring back this fall after mostly disappearing for the previous two years — looks like it's finally receding in most places, according to the latest data out Friday from the CDC.

"In a couple areas, we're seeing activity increase or plateau," Dr. Shikha Garg, a medical epidemiologist at the CDC, told NPR in an interview Friday. "But in most areas, it's been declining."

The virus posing the biggest threat right now is — you guessed it — the one that causes COVID-19.

COVID "concerns us most" in the days and weeks ahead

"We're seeing sustained increases of COVID infections across the nation," Dr. Ashish Jha, the White House coronavirus response coordinator, told NPR in an interview. "So COVID is the thing that concerns us most as we look at the days and weeks ahead."

The rate at which the coronavirus is being detected in wastewater, which has become a bellwether for the pandemic, has tripled or quadrupled in many parts of the U.S. in recent weeks, Jha says. COVID-19 hospitalizations have jumped 70%, he says. And 300 to 400 people are dying every day from COVID-19.

To make matters worse, all this is happening as yet another new, even more transmissible variant has taken over in the United States. Called XBB.1.5, this new omicron subvariant was barely on the radar in late November. But according to new estimates released Friday by the CDC, XBB.1.5 now accounts for almost a third of new infections and is the dominant variant in the Northeast.

"The current increase in cases that we are seeing really began around the Thanksgiving holiday when people gathered. And as we went into the bigger holiday season — the Hanukkah/Christmas holiday season — that has accelerated infections further," Jha says.

Because "every major holiday has led to a bump in cases throughout the entire pandemic, it stands to reason that we're going to see a clear increase in infections, and cases and hospitalizations, unfortunately, over the next few weeks," he says.

Why omicron subvariant XBB.1.5 is spreading so fast

The prevalence of the coronavirus's omicron subvariant XBB.1.5 "shot up like a rocket," says Sam Scarpino, who has been tracking new variants at Northeastern University. "This variant has displaced other variants in a way that we've never seen before. That's kind of alarming."

The good news is that so far there's no evidence the new variant makes people sicker than earlier versions of the coronavirus. And the immunity that people have from getting infected and vaccinated should protect most from getting really sick. So no one thinks this winter will be anything like the first two horrific pandemic winters.

But XBB.1.5 can partially sneak around immunity as easily as anything before it. And it has developed something none of its predecessors had: a mutation that lets it infect cells more easily than the others. That makes this version of COVID-19 even easier to catch.

"XBB.1.5 has gotten a specific mutation that enables it to maintain antibody escape properties while also giving it an advantage for spreading through the population," says Jesse Bloom, a virologist at the Fred Hutchinson Cancer Center in Seattle who has been studying variants.

That means lots of people are probably going to get COVID-19 this winter.

"The question is not whether it's going to cause a surge. It almost certainly will. The question is: How big is the surge going to be?" Scarpino says.

So public health authorities are once again urging people to protect themselves.

How to protect yourself from coronavirus subvariant XBB.1.5

"What is clearer now, compared to even a year ago, is that we can really blunt the worst of it by doing the things that we know work," Jha says.

That includes getting vaccinated and boosted, especially if you're older. Most deaths from COVID-19 are occurring in people age 65 or older.

Other precautions include avoiding crowded, poorly ventilated parties, restaurants, bars and other places; testing before gathering; and, yes, putting that mask back on in risky situations. And if you do get sick, check with your doctor about getting treatment quickly.

"It is a time not to let your guard down," warns Dr. Tina Tan, an infectious disease specialist at Northwestern University.

Fortunately, most of the precautions that lower your risk of catching COVID-19 will also help protect you against any resurgence of RSV or the flu. The winter is still young, and the flu is still circulating at fairly high levels in many places, experts note, which means many people are still suffering through fevers, body aches, chills and other symptoms. And the holidays may have jump-started more infections that will continue to surface and spread in the coming days as people return home from trips and family gatherings, schools reopen and people go back to work.

The U.S. could see another flu wave later this winter. That's what happened in some parts of the Southern Hemisphere's winter, and it often occurs in the Northern Hemisphere too.