(CNN) The coming flu season may be a doozy.
Even as coronavirus was devastating populations around the world, killing 3.7 million people globally, doctors and public health officials noticed something else was missing: There was almost no flu.
One child died from flu this year in the US. In 2019-2020, there were 199 flu-related deaths in children and 144 the season before that. Flu cases, usually counted in the tens of millions, only accounted for a few thousand this year in the US.
"Flu hasn't been anywhere, with the exception of some reasonable activity in western Africa," said Richard Webby, an influenza specialist at St. Jude Children's Research Hospital in Memphis.
"No one has seen it. That includes countries that have done lockdown. It includes countries that haven't done any lockdown. It includes countries that have done a good job controlling the pandemic. It includes countries that haven't done a good job," Webby told CNN.
It's not entirely clear why. Many experts believe that measures taken to help control coronavirus also prevented the spread of influenza. It's also possible that coronavirus somehow outcompeted or interfered with flu.
Either way, Webby and other experts think the lull in flu activity is only temporary. They worry that when influenza returns, likely this fall, it will be with a vengeance.
"The worst flu season we ever had may be coming," Webby said.
"When it comes back, it is going to be a doozy of a season," agreed Aubree Gordon, an epidemiologist who studies flu at the University of Michigan School of Public Health.
One reason the coming influenza season is likely to be a bad one can be explained by human behavior. People tired of lockdowns, of wearing masks, of staying away from other people, will want to celebrate the freedom offered by vaccines that protect them from coronavirus and the waning of the pandemic.
They may overdo it.
Travel is already on the increase, restaurants are filling back up, and schools are planning to re-open with in-person classes.
But while people flocking to resorts, bars and family gatherings may be much safer from coronavirus, they're not any safer from flu or other respiratory viruses that are spread in the same ways that coronavirus is: in the air, in droplets and on surfaces.
"I do think with a greater number of individuals not wearing masks and not as much social distancing, there is definitely going to be an uptick in the common respiratory infections that we see seasonally," Allison Aiello, who studies the spread of infectious diseases at the University of North Carolina's school of public health, told CNN.
Aiello says North Carolina is already seeing an increase in respiratory diseases.
"We should expect there to be some increases, especially in the fall as children go back to school," she said.
"It's not just flu. It's all the other respiratory viruses," Webby said. These include not only influenza, but respiratory syncytial virus or RSV, adenoviruses, the coronavirus strains that cause the common cold, rhinoviruses and others.
"I certainly think as the mitigation measures as we have in place for Covid come down and kids go back to school in person and we all start traveling again, particularly internationally, we know all sorts of respiratory viruses are going to have much more opportunities to spread," Lynette Brammer, who leads the US Centers for Disease Control and Prevention's Domestic Influenza Surveillance Team, told CNN.
"And we certainly expect that flu and all the other respiratory viruses that have been low over the last year will come back," she added.
"In certain ways, we have returned back to normal. You start putting kids together and you will get viruses."
However, Brammer is cautious about making predictions.
"Flu is always unpredictable, and I feel like right now it's more true than ever," Brammer said.
There's a second reason to think the 2021-2022 influenza season might be a bad one. There's a theory, not well documented, that the human body's immune response is naturally boosted by repeated, annual exposures to viruses such as flu. These exposures might not be enough to make people sick, but they're enough to remind the immune system to keep up its defenses.
"The longer you go without exposure, the more likely you are to be symptomatic and more likely to be sicker," Gordon said.
"We do know the longer you go without being exposed to influenza, the more symptomatic you are. Sicker individuals lead to more severe cases. We absolutely know that."
The same goes for RSV, non-Covid-19 coronaviruses and other infections. "I would kind of generally be worried about all of them. All of them can cause severe disease. All of them can cause pneumonia," Gordon added.
RSV, especially, takes a toll on babies and very young children. It kills an estimated 100 to 500 children every year, and 14,000 adults, mostly over the age of 65.
Many of the 4 million or so infants born during the pandemic will be getting their first exposures to RSV and other viruses as they go into daycare for the first time ever. "We don't know what effects will be of all these young children delaying their first exposure of RSV," Gordon said.
"There probably are going to be very large RSV epidemics."
Aiello is less certain about the possible effect of avoiding germs for a year or so. "This is a short period of time," she said. Several years of avoiding exposure may be expected to have an effect, but the 15 months or so most people have been social distancing, working from home or staying out of classrooms may not have been long enough to affect immune systems.
But the fall respiratory flu season may feel worse, even if it actually isn't, Aiello said. If nothing else, many children will be packing two years' worth of exposure to a range of viruses into a single season.
"When an individual hasn't been sick for a while, it may seem like you are experiencing more robust symptoms," she said.
Flu will be the one virus that get measured. Doctors do not test people for most of the other respiratory viruses -- mostly because there's no specific treatment for them -- but the CDC tracks influenza.
Flu kills between 12,000 and 61,000 people a year, depending on the season, the CDC says.
It says the 2019-2020 season was a moderate one in which 38 million people in the US got sick with flu, 18 million saw a health care provider for treatment, 400,000 were sick enough to be hospitalized and an estimated 22,000 died.
About 8% of the US population gets sick from flu each season, with a range of between 3% and 11%, depending on the season, the CDC says.
Much will depend on how many Americans get vaccinated. Each year, just under half the population gets a flu vaccine, even though the CDC recommends an annual flu shot for almost everyone over the age of 6 months.
One thing the CDC knows for sure: Flu activity is impossible to predict.
"I don't know what to expect. I don't know," Brammer said. "We are just going to have to wait and see."
Brammer has seen every flu season for decades, and each one is unique.
"Every time you think you know what will happen, it will do something totally different," she said.