In a way, it’s hard not to admire influenza.

It is a tenacious little Tom Sawyer of a bug that persuades its victims to do all of its work for it and constantly reinvents itself to ensure it will have a never-ending supply of hosts.

“From the evolutionary perspective, they’re pretty good,” said Michael Dennis, an associate professor of biology at Montana State University Billings.

The influenza virus has bedeviled humans for centuries, and chances are that it is with us for good.

Even the best scientific minds cannot predict how influenza will change or prevent it from doing so, and that means it cannot be eradicated.

“Every time it creates itself, it rolls the dice,” said Dr. David Graham, an infectious-disease specialist at Billings Clinic. “It is creating itself billions and billions of times. We just don’t have the ability to say it’s going to be this one-in-a-trillion chance or this one-in-a-trillion chance.”

Because of its ubiquitous presence, influenza tends to play second fiddle to other health concerns. But every so often — 1918, 1957, 1968, 2009 — it explodes onto the public consciousness.

“People don’t panic until it becomes a pandemic,” said Becky Wolfe, head of infection control and prevention at St. Vincent Healthcare. “But we have people every year die from the flu.”

Influenza kills 36,000 Americans, including 150 to 200 Montanans, in a typical flu season.

Dennis likes to call the influenza virus a piece of bad news wrapped in protein.

“It’s not even a lot of news,” he said. “But it’s enough to get this agent replicating to make a lot of itself.”

A flu virus is a strand of eight chromosomes, each containing 10 genes, wrapped in a blob of lipid material.

The influenza A virus, which causes most flu outbreaks, wasn’t isolated until 1933, but scientists and historians have found evidence of flu dating back thousands of years. Still, its origin is a mystery.

“Here’s what people don’t want to hear: Scientists don’t know where viruses came from,” Dennis said.

Influenza viruses are tiny — 250,000 of them placed end-to-end would measure just an inch — and they need a living host to survive. Scientists disagree about whether viruses themselves are alive.

“They’re on the fringes of what we consider living,” Dennis said. “They don’t metabolize. They don’t trap energy. We like to say about living things that they eat and they reproduce. Viruses reproduce, but they don’t trap energy. They get a cell to do everything.”

Viruses enter individual cells in their hosts and take over, persuading the cells to replicate their invader.

The 2009 H1N1 influenza virus settles in cells deep inside the human respiratory system, but flu viruses can also take up residence in pigs, birds and some other animals.

Within a day or two, the replication process kills the hijacked cell, releasing the newly produced viruses inside the host. Those viruses enter other cells, and the process starts over.

The host’s body tries to stop the spreading virus, but it is not always successful.

“Just like if someone were to come into my house, I would fight back,” Wolfe said. “The body, in order to get rid of it, causes coughing and sneezing. To get rid of it, you spread it to others.”

Fever, a hallmark symptom of influenza, also serves a purpose, Wolfe said.

“The fever I get is my body’s army to fight infection,” she said. “When I have a fever, I send an army of white blood cells to attack the virus.”

Sometimes the body can send a virus packing, and sometimes it can’t. Many factors play into that equation, including how healthy the host is and, probably, how her genes interact with the virus’ genes.

“We don’t understand why two people who are totally healthy, one will get it and one will not,” Wolfe said.

The body remembers viruses it has encountered. When faced with them again, it can fight them off. That’s known as immunity, and it can be achieved through vaccination as well as through infection.

If a host cannot fight off a virus, she becomes vulnerable to other infections. Most people who die after contracting influenza actually are killed by bacterial infections made possible because the body has been weakened by the flu.

In 1918 and 1919, at least 20 million people around the world, and 675,000 in the United States, died during an influenza pandemic.

It was an outbreak of H1N1 influenza, and it struck healthy children and young adults particularly hard.

Viruses had not yet been discovered, leaving medical personnel to guess at how the disease spread and how it should be treated.

By the time the next flu pandemic struck, killing 70,000 Americans in 1957 and 1958, scientists knew a virus caused influenza and had developed a vaccine, first licensed in 1944, to prevent infection.

But the vaccine did not always work, and it still doesn’t. That’s because influenza changes all the time.

“The virus is pretty good at evolving,” said Dr. Steve Helgerson, the state medical officer for Montana. “It keeps changing and finding ways to infect.”

Some viruses, such as chickenpox or polio, never change. The chickenpox virus circulating today is the same one that infected children 100 years ago.

But the flu virus circulating now, H1N1, is far different from the strains of flu that were present last year and even differs from the H1N1 strain that killed so many people in 1918.

Scientists call those changes, which occur when a virus’ genes get mixed up or reassorted during reproduction, drift and shift.

Drifts are small changes, and they happen constantly. A new flu vaccine is offered every year because of drifting. Shifts are bigger, rarer changes.

“They occur not too frequently, but when they do we tend to notice,” Helgerson said.

When the influenza virus shifts, it can quickly reach pandemic levels because people have not been exposed to the reassorted virus before. That’s probably what happened in 1918, and it is what’s happening now.

The 2009 H1N1 influenza is the seventh new strain of flu to be identified since 1977.

“Many people, especially people under 50 years of age, haven’t had the opportunity to respond to a virus like this,” Helgerson said.

But not every shift is deadly or even virulent. The new version of the virus has to be able to spread between humans, and often something in its genetic makeup prevents that from happening.

H5N1 influenza, the so-called “avian flu,” is a shifted form of the virus that scared a lot of people when it was detected in 1997. But, so far, H5N1 has not been able to spread from person to person.

“There are new viruses formed every day, but most of them are duds,” Graham said.

H5N1 can spread from birds to humans, and it is influenza’s ability to infect multiple species, as well as its mutabilty, that guarantees its survival.

Polio or smallpox could be eliminated from the modern world because they are static viruses that affect only humans. Vaccinate enough people, and the virus would disappear.

But influenza probably won’t disappear because it can infect pigs and birds. It is often inside those animals that the virus changes, especially when two strains enter the same host.

Scientists pay attention when a new strain of influenza surfaces because it could kill thousands or even millions of people.

But it could turn out to be like this year’s H1N1, which has made a lot of people sick but hasn’t been a killer compared with other flu strains.

In the 2003-04 flu season, for instance, 152 American children died from an H3N2 strain of influenza.

So far this fall, 73 children have died from H1N1 influenza, including one in Billings. Another 56 died last spring.

The 2009 H1N1 influenza might still turn more deadly — pandemics tend to come in three waves, and we’re in H1N1’s second wave now — but, then again, it might not.

“It’s possible there will never again be anything like the 1918 pandemic,” Helgerson said.

Contact Diane Cochran at dcochran@billingsgazette.com or 657-1287.