A surge in respiratory infectious illnesses is continuing in China, particularly among children. A researcher says a probable cause of the outbreak is that many people do not have sufficient immunity following thorough anti-infection measures implemented under the government's zero COVID policy.

Since October, the number of patients with respiratory illnesses has been rapidly rising.

Many children wearing masks were seen visiting a key pediatric hospital in central Beijing on Wednesday morning.

The National Health Commission said on Sunday that the recent respiratory diseases are mainly caused by the influenza virus.

The commission also pointed to other pathogens, such as rhinovirus, which causes the common cold, and mycoplasma pneumonia, which causes fever and coughing.

The World Health Organization says Chinese authorities have informed it that no new pathogen has been detected in the recent outbreak.

Professor Wu Zhiwei at Nanjing University's Medical School told state-run TV that one factor behind the increase may be that many people have not been infected with other viruses in the past three years due to China's anti-COVID measures. He indicated this has caused low immunity levels.

Health officials are calling on people to be vaccinated, wear masks and take other anti-infection steps.