(CNN)Dr. Adrian Burrowes has seen hundreds of Covid-19 patients. But he's especially worried about what will happen this flu season -- even more so than last year.
This fall and winter could mark the first surge of patients infected with both the flu and the Delta variant -- the most contagious strain of coronavirus to hit the US.
"You can certainly get both the flu and Covid-19 at the same time, which could be catastrophic to your immune system," said Burrowes, a family medicine physician and assistant professor of family medicine at the University of Central Florida.
"I'm definitely more concerned this year than I was a year ago," he told CNN. "And the reason why is because now we have pretty significant Covid fatigue amongst the American population."
Heightened safety precautions against Covid-19 last year helped stymie the spread of the flu last year, said Dr. Mark Rosenberg, president of the American College of Emergency Physicians.
"Last year, more people staying home and covering their faces when they did go out kept flu numbers historically low," Rosenberg said in a written statement this month. But "this year the situation could be dangerously different."
With more people out and about -- and millions of Americans not vaccinated against the flu nor Covid-19 -- "I do believe you're going to see a rise in flu cases," Burrowes said.
"I do believe you're going to see coinfection with flu and coronavirus. And I do believe you're going to see a higher rate of mortality as a result of that."
What Covid-19 and the flu together could do
On their own, both Covid-19 and the flu can attack the lungs, potentially causing pneumonia, fluid in the lungs or respiratory failure, the US Centers for Disease Control and Prevention said.
Each illness can also cause sepsis, cardiac injury and inflammation of the heart, brain or muscle tissues.
Having both illnesses simultaneously "would increase the risk of longer-term effects of any of those organ systems," said Dr. Michael Matthay, a professor of medicine and a critical care specialist at the University of California, San Francisco.
"The two together definitely could be more injurious to the lungs and cause more respiratory failure," Matthay told CNN last year and reiterated this week.
Respiratory failure doesn't necessarily mean your lungs stop working. It means the lungs can't get enough oxygen into the blood.
"Acute respiratory failure can be a life-threatening emergency," the National Heart, Lung, and Blood Institute says. "Respiratory failure may cause damage to your lungs and other organs, so it is important to get treated quickly."
But with so few flu cases last year, there's not much data on how many people had the flu and Covid-19 simultaneously. "Health experts are still studying how common this is," the CDC said.
Internal medicine physician Dr. Jorge Rodriguez typically sees about 60 flu patients each flu season. Last season, the Los Angeles doctor saw zero.
But this year, "the odds of a double whammy are definitely going to increase," Rodriguez told CNN this week. "The fever may be worse. The shortness of breath may be worse. The loss of smell and taste could be worse. And on top of all that, it could last longer."
Both he and Burrowes said they expect to see more patients with both illnesses in the coming months. Not only are more people ditching last year's safety precautions, but having one virus can weaken the immune system's ability to respond to a second virus.
Think about it this way, Rodriguez said: "If you get hit with a hammer, it will hurt. But if you already have a broken leg and get hit with a hammer again, it will hurt more. And it will take longer to heal."
That's particularly worrisome because some Covid-19 patients are already struggling with monthslong complications.
While flu symptoms often subside within a few weeks, Rodriguez and other doctors are seeing more patients with long Covid.
Increasingly, they include young, previously healthy adults, Rodriguez said.
He recalled a "38-year-old, super healthy" athlete and martial artist who came down with Covid-19. A year later, he still can't exercise regularly.
"He just does not have the pulmonary reserve to do it anymore," Rodriguez said. "It's affected (other aspects of) his health. He's gained weight. He's depressed because he's not the same person that he feels he used to be."
On the other side of the country, Burrowes has also noticed an increase in younger Covid-19 patients.
"This year, I have seen over 400 patients with Covid," he said. "And since April, the overwhelming majority of those patients have been under the age of 40. Of those patients, 98% were unvaccinated."
How a one-two punch could hit health care
In some states, hospitals are already overwhelmed with Covid-19 patients. Some have activated crisis standards of care, allowing hospitals to ration resources and treatment.
Add an influx of flu patients this season -- or flu and coronavirus patients -- and the situation could get dire, Burrowes said.
Not only could that reduce other people's access to prompt health care, it can also endanger medical workers -- which, in turn, would also impact the public.
"When people say coronavirus is just like the flu, that bothers me," he said. "I have never lost a physician colleague to the flu. ... But I've lost four physician colleagues, relatively healthy, that all died from coronavirus."
Each of those colleagues died before Covid-19 vaccines were available, Burrowes said.
But even now with vaccination and protective equipment, "we're treating patients and we're right up against them and we're in close proximity with them for prolonged periods of time," he said.
The public needs to understand that if they don't help mitigate the spread, they may get a harsh reality check if they have a car accident or another emergency and don't get the urgent care they're expecting, Rodriguez said.
How to avoid a flu-Covid-19 double whammy
"Getting vaccinated is one of the best ways to avoid the double whammy," said Rosenberg, the head of emergency physicians' association.
But only 55.8% of Americans have been fully vaccinated against Covid-19, according to CDC data Tuesday.
And "only about half of Americans get an annual flu vaccine," the CDC said. Most children who die from the flu are not vaccinated.
The CDC recommends flu vaccines for everyone ages 6 months and up, with rare exceptions. Covid-19 vaccines are available to everyone ages 12 years and up, and children ages 5 to 11 might become eligible in the next several weeks.
Conveniently, Americans can now get a Covid-19 vaccine and a flu shot at the same time, the CDC said. That could spur more people to get both shots, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.
"If that means going in and getting the flu shot in one arm (and) the Covid shot in the other, that's perfectly fine," Fauci said.
While breakthrough infections are possible with both the flu and Covid-19 vaccines -- which work differently in the body -- "both of those vaccines have that same ability to decrease the severity of your illness," Burrowes said.
Even though he's fully vaccinated, Rodriguez plans to wear a mask in public this fall and winter to help minimize the chances of a breakthrough infection -- either of coronavirus or the flu, he said.
He doesn't want to risk reducing his immune system and becoming vulnerable to the other virus.
"I don't have a problem with taking extra safety precautions," he said. "My freedom is not affected in the least."
