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.

And the virus had split into two lineages — called Yangtze and Pearl, after the river deltas in which each was spreading — complicating efforts to make vaccines.

In October, the World Health Organization put out an update citing new cases of H7N9 infection as cold weather set in and noting that poultry farmers were vaccinating flocks against both this virus and other strains.

At about the same time, a well-known virologist at the University of Wisconsin-Madison showed that a Chinese H7N9 strain could both kill ferrets and be transmitted between them.

Because ferrets suffer roughly the same effects from flu that humans do, the development was “not good for public health,” said the virologist, Dr. Yoshihiro Kawaoka.

Many microbiologists consider influenza to be the virus most likely to start a pandemic that kills millions, as the 1918 Spanish flu did. But the flu is notoriously unpredictable. Public health experts have become wary about raising alarms over new strains because the grave predictions made in 2005 and 2009 turned out to be overblown.

In 2005, it was feared that the H5N1 avian flu, which killed or forced the culling of millions of chickens and ducks, would mutate and spread widely among humans.

It still circulates, primarily in Egypt and Indonesia, but so far has not become a human epidemic. As of last month, only 860 people in 16 countries had tested positive for the infection. Still, more than half of them died.

And in 2009, a new H1N1 flu virus containing genes from both American and Eurasian pigs emerged in Mexico, prompting scary “swine flu” headlines and the declaration of a health emergency.

That virus has now become one of the seasonal flu strains circling the world. It has infected millions but has killed relatively few people.

The 2017-2018 flu season in the United States does not yet seem unusually threatening. But Australia, where winter recently ended, just suffered one of its deadliest outbreaks in a decade, and the H3N2 and B Yamagata strains that dominated there are now the most common ones in the United States.

Flu hospitalizations in this country rarely shoot up before mid-December, and Americans are far more likely than Australians to get flu shots.

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