As China continues to monitor an increase in cases of human metapneumovirus (HMPV), here's what you need to know about the illness.

The WHO said Chinese data up to Dec 29 showed detections of HMPV, seasonal influenza, rhinovirus, and respiratory syncytial virus (RSV) have all increased in recent weeks, particularly in northern parts of China. Influenza is currently the most commonly reported cause of disease, it said.

While cases are climbing in China, the situation is very different from what it was when Covid-19 emerged five years ago, medical experts say. HMPV has circulated in humans for decades.