That's because the first line of defense in the immune system, called the innate response, is nonspecific — meaning it is not tailored to any specific type of virus.
Infected cells respond by producing antiviral proteins called interferons, among other weapons, and the interferons that help ward off one type of virus generally will help ward off another.
That doesn't mean it's a good idea to get a second infection on purpose. While a second virus may not increase your misery, it certainly can't make you feel any better.
And in plenty of circumstances, the second infection can make matters worse, said te Velthuis, the Princeton scientist.
That could happen if the viruses infect different parts of the body, for example. Influenza typically invades the upper airways, while some strains of the coronavirus can replicate in the lower airways and the digestive tract, he said.
"The combined infection may trigger immune responses in several locations and a stronger feeling of malaise overall," he said.
All sorts of variables come into play, such as the person's prior exposure to either of the two viruses, whether or not they've been vaccinated, and the sheer number of viruses they ingest, said Penn State's Cattadori.
The impact of a coinfection can be even worse when the second infection is bacterial. Flu and COVID both cause temporary damage to the lining of the airways, impairing the patient's ability to defeat bacteria, said Temple's Fekete.
"The bacteria have an easier time getting in," he said.
In an older person or someone who smokes cigarettes, he said, the result can be pneumonia.
WHAT TO DO IF YOU HAVE 'FLURONA'
Most people with the flu and COVID may not realize it, as out-of-hospital testing for the flu is unusual. And lately, as we've all heard, COVID tests are in short supply.
But whether you're infected with one virus or the other, or both, the usual advice applies:
Stay away from others. Drink plenty of water. Call your doctor, and seek emergency care if you feel difficulty breathing.
Treatments are available for both types of infections, should your doctor recommend them.
Above all, do not panic, especially if you have been vaccinated against one virus or both. While the vaccine may not have prevented infection in your case, it is very likely to prevent severe disease.©2022 The Philadelphia Inquirer, LLC. Visit at inquirer.com. Distributed by Tribune Content Agency, LLC.