The most important trade war to come may have nothing to do with cars, steel or soybeans. It may involve a virus.

The vast agricultural enterprise in southern China is the source of most new flu strains. Under rules established by the World Health Organization in 2011, China has routinely shared samples of them with researchers in the United States and other countries where vaccines are developed. But recent trade tensions may be leading it to change course.

Influenza is a serious disease. The seasonal variety kills thousands of people worldwide each year. But its impact pales in comparison with pandemic flu. That is the kind that emerges every few years and spreads like wildfire around the globe, sometimes killing millions – as it did in 1918.