Bird flu has been spreading. Though not a worry right now for people, it could become one. Here’s why.

A bird flu outbreak at a mink farm has reignited worries about the virus spreading more broadly to people.

Scientists have been keeping tabs on this bird flu virus since the 1950s, though it wasn’t deemed a threat to people until a 1997 outbreak in Hong Kong among visitors to live poultry markets.

As bird flu hits more and varied animals, the fear is that the virus could evolve into a form in which it might spread more easily among people — and potentially trigger a pandemic.

For now, the risk to the public is low, says Dr. Tim Uyeki of the federal Centers for Disease Control and Prevention.

But bird flu has been a serious problem in the past. Scientists say another kind of bird flu was likely behind the devastating 1918-1919 flu pandemic, and avian viruses played roles in other flu pandemics in 1957, 1968 and 2009.

Here are some facts about the bird flu virus and why it’s getting renewed attention:

WHAT IS BIRD FLU?

Some flu viruses mainly affect people. Others chiefly occur in animals. For example, there are flus that occur in dogs and also pig — or swine — flu viruses. And there are avian viruses that spread naturally in wild aquatic birds like ducks and geese, then to chickens and other domesticated poultry.

The bird flu virus drawing attention today — Type A H5N1 — was first identified in 1959 by investigators looking into a flu outbreak in chickens in Scotland. Like other viruses, it has evolved, spawning newer versions of itself.

By 2007, the virus was found in more than 60 countries. In the United States, it recently has been detected in wild birds in every state and in commercial poultry operations or backyard flocks in 47 states.

Since the beginning of last year, tens of millions of chickens have died of the virus or been killed to stop outbreaks from spreading — one of the reasons cited for soaring egg prices.

HOW OFTEN DO PEOPLE GET BIRD FLU?

The Hong Kong outbreak in 1997 was the first time this bird flu was blamed for severe human illness. Of 18 people infected, six died.

To contain that outbreak, the Hong Kong government closed live poultry markets, killed all of the birds in the markets and stopped bringing in chickens from southern China. For a while, that worked.

Symptoms are the similar to that of other flu infections, including cough, body aches and fever. Some people don’t have noticeable symptoms. Others develop life-threatening pneumonia.

Globally, nearly 870 human infections and 457 deaths have been reported to the World Health Organization in 20 countries. But the pace has slowed. There have been about 170 infections and 50 deaths in the past seven years. In most cases, infected people got it directly from infected birds.

The first and only U.S. case occurred last April. A prison inmate in a work program picked it up while killing infected birds at a poultry farm in Montrose County, Colorado. His only symptom was fatigue. He recovered.

CAN IT SPREAD AMONG PEOPLE?

In some instances, investigators concluded that the bird flu virus apparently spread from one person to another. That happened in Thailand, Vietnam, Indonesia, China and Pakistan, most recently in 2007.

In each cluster, it spread within families at home. Scientists don’t think it can spread easily through casual contact, as seasonal flu can.

But viruses mutate. Scientists worry about the growing number of opportunities for bird flu to mix with other flu viruses in infected people or animals and mutate, making it easier to spread to people.

It wouldn’t take much for that to happen.

“And then we would be in a really tough situation,” says Dr. Luis Ostrosky, chief of infectious diseases and epidemiology at the University of Texas Health Science Center at Houston.

Uyeki says he was most worried about H5N1 during the earlier clusters but that thist kind of human-to-human spread doesn’t appear to be happening now.

WHAT HAPPENED AT THE MINK FARM?

Recent concern among public health experts has been fueled by the detection of infections in a variety of mammals. The growing list includes foxes, raccoons, skunks, bears and even marine mammals such as seals and porpoises. Officials in Peru said three sea lions found dead in November tested positive for bird flu, and the recent deaths of hundreds of others could be due to bird flu.

Last month, a European medical journal reported on a bird flu outbreak in October at a mink farm in Spain. The mink were fed poultry, and wild birds in the region had been found to have bird flu. Researchers said they think the virus then spread from mink to mink — a worrisome scenario. No workers were infected.

Jennifer Nuzzo, director of the Pandemic Center at the Brown University School of Public Health, says the outbreak virus is being watched for mutations that could make it more easily transmitted to people and potentially between people.

“That’s the real worry,” Nuzzo says.