The Netherlands is experiencing a bizarre uptick in pneumonia cases among children, marking the second country to report an outbreak of this type this week.

The Netherlands Institute for Health Services Research (NIVEL), a research institute in Utrecht, around 25 miles south of Amsterdam, reports that 80 of every 100,000 children between ages 5 and 14 came down with pneumonia last week.

This is the largest outbreak of pneumonia NIVEL has recorded in recent years. At the peak of the 2022 flu season, when pneumonia cases were most common, there were 60 recorded cases for every 100,000 children in the age group.

A news outlet in the Netherlands said neither NIVEL nor the National Institute for Public Health and the Environment, the Dutch equivalent of the U.S. Centers for Disease Control and Prevention (CDC), could give an explanation as to why pneumonia cases were increasing.

Mysterious pneumonia cases in China have also begun to raise alarms. First reports emerged last week that children’s hospitals in Beijing and the province of Liaoning were overrun by children coming in with pneumonia.

Chinese officials told the World Health Organization that no new pathogens were detected in the outbreak, and instead the illnesses were caused by known seasonal viruses such as the flu and RSV, along with the bacteria Mycoplasma pneumonia.

Officials said that strict COVID measures, which were lifted at the end of 2022, left the population vulnerable to these annual viruses. Now, during the first flu season since the lifting of the nation’s strict COVID lockdowns, the population is being ravaged by the annual bugs.