A small group of Republican senators on Friday called on President Biden to ban travel from China to protect against an outbreak of respiratory illnesses in children there, even as scientists and global and American health officials said there were no signs of a threatening new pathogen.

Instead, those experts said, the evidence so far pointed to a surge of age-old infectious agents such as influenza, driven by the colder weather and China’s emergence from stringent Covid lockdowns. The World Health Organization said last week that China had shared data about its outbreak, including laboratory results from infected children, that did not show any unusual pathogens.

Dr. Mandy Cohen, the director of the Centers for Disease Control and Prevention, echoed that assessment on Friday. She said American officials had also been in touch with Chinese authorities, academic experts and health workers.

“What we have all been able to ascertain is that there is no novel pathogen,” she said. “This is all related to upticks of known viruses and bacteria in their pediatric population.”

What is sickening children in China?

The W.H.O. said that the data China shared last week showed a rise in the number of children hospitalized with cold and flu viruses and respiratory syncytial virus, or R.S.V., since October. The agency also said infections with mycoplasma pneumoniae, a bacterium that generally causes mild illness, had been climbing since May.

Those illnesses, together with Covid, accounted for a surge of respiratory disease in children, said Chinese health officials, who announced the rise in cases at a news conference in mid-November. Parents there have described waits of eight hours or more at children’s hospitals.

Some of the same pathogens are already on the rise in the United States as winter takes hold, experts said, obviating the threat of imported cases from China.

Mycoplasma pneumoniae, which sickens millions of Americans every year, tends to cause an increase in infections every three to seven years for reasons that are not well understood.

“There isn’t a place in the world that doesn’t have mycoplasma, that doesn’t have rhinovirus and influenza virus,” said Dr. William Schaffner, a professor of infectious diseases at the Vanderbilt University School of Medicine. “They are the normal respiratory viruses and germs that we encounter as we normally grow up.”

Why the surge?

As countries have lifted Covid lockdowns in recent years, they have tended to experience surges of routine respiratory illnesses in children.

That was true last year in the United States, where an earlier-than-expected wintertime “tripledemic” of Covid, flu and R.S.V. cases strained hospitals. Before that, Australia, too, suffered an especially bad flu season. Even this year’s respiratory illness season in the United States started a bit earlier than usual, Dr. Schaffner said.

Scientists attribute those outbreaks to children’s immune systems being behind schedule after years of lost learning about how to fight off everyday viruses during Covid lockdowns. Kept from schools and day care centers, they were sheltered from pathogens that would otherwise have infected them sooner.

“Then when they all got together,” Dr. Schaffner said, “whoo, the viruses had a field day.”

Children in China were especially vulnerable because of the length and severity of lockdowns there, scientists said, potentially creating a more substantial post-lockdown boom in routine infections.

Scientists have also reported elevated rates of resistance to mycoplasma pneumoniae antibiotics in China, a problem that could sharpen the severity of outbreaks and that W.H.O. officials said they were trying to learn more about.

Is the United States having any unusual outbreaks?

A county in Ohio this week reported “an extremely high number of pediatric pneumonia cases,” including some caused by cold viruses or mycoplasma pneumoniae. But health officials there said they did not believe that a new respiratory disease was responsible or that there was any link with outbreaks abroad.

Dr. Cohen, the C.D.C. director, said that more than 80 percent of emergency rooms across the country make daily reports to health officials. In those reports, she said, “we are not seeing anything that is atypical in terms of pneumonia-related emergency department visits.”

Ohio has not reported any deaths as part of its pediatric outbreak, Dr. Cohen said, and most children are receiving treatment and recovering at home.

“Hospital capacity is fine, children are recovering at home — these are pathogens that are known to us,” Dr. Cohen said.

What can Americans do?

“Don’t fret about what’s happening in China,” Dr. Schaffner said. But, he said, “This afternoon, get yourself vaccinated.”

Even if the outbreaks in China do not appear to pose a global threat, scientists said, familiar respiratory illnesses were spreading widely enough in the United States that those eligible should get vaccinated against Covid, the flu and R.S.V.

By mid-November, fewer than four in 10 adults in the United States had received flu vaccines. Uptake of the updated Covid vaccines, too, has been sluggish.

Meanwhile, American and global health officials said they would keep monitoring cases in China, which has faced intense scrutiny after covering up early cases of both the SARS virus in 2003 and Covid.

While China has been more forthcoming about its latest outbreaks than about early Covid cases, Dr. Schaffner said, it could provide more public data about the age of those infected, the cities where illnesses were spreading and the causes of reported cases of severe pneumonia.

Travel bans, though, would not help, Dr. Schaffner said. The W.H.O. has also said travel restrictions were not warranted.

Sheryl Gay Stolberg contributed reporting.