While trying to avoid alarmism, global health agencies are steadily ratcheting up concern about bird flu in Asia. Bird viruses that can infect humans — particularly those of the H7N9 strain — continue to spread to new cities there.

Since October 2016, China has seen a “fifth wave” of H7N9 infections. Nearly 1,600 people have tested positive, almost 40 percent of whom have died.

Most had been exposed to live poultry, but a small number of clusters suggest that the virus could be passing from person to person.

In September, the Centers for Disease Control and Prevention summarized some disturbing developments. The H7N9 virus had become lethal to birds, which made it potentially more dangerous to people but also easier to spot.