Dense populations of animals can help speed the mutation rate of viruses such as influenza. (Kiichiro Sato/Associated Press)

More than 100 people have died in Mexico as a result of an outbreak of swine flu, a strain of the influenza virus that normally targets pigs but has occasionally mutated enough to infect and spread in humans.

This ability to mutate from one host to the next, or one species to the next, is one of the traits that has given the influenza virus a long life and made it both nearly impossible to eradicate and potentially dangerous to animals and humans alike.

Here we explore the reasons why viruses mutate, how they do it, and what impact their environment plays in their ability to cause pandemics.

Why do viruses mutate?

To survive: unlike plants, animals and other organisms, the only way a virus can reproduce is through a host cell, which it does by attaching its surface proteins to the cell's membrane and injecting its genetic material into the cell. This genetic material, either DNA or RNA, then carries with it the instructions to the cell's machinery to make more viruses. These new viruses then leave the cell and spread to other parts of the host organism.

But host organisms are not passive observers to this process, and over time a human's or pig's immune system can learn from these encounters and develop strategies to prevent reinfection. The next time the same virus comes to a host cell, it may find that it is no longer able to attach to the cell's surface membrane. So to survive, viruses must adapt or evolve, changing its surface proteins enough to trick the host cell into allowing it to attach.

What makes one virus mutate quickly while others change more slowly?

The genetic material inside the virus plays an enormous role in how quickly a virus mutates, which in turn can impact how the illness can spread in the population.

Viruses that replicate through DNA use the same mechanisms the host cell uses to create its own DNA, a process that includes a kind of "proof-reading" of the genetic material being copied. This means mutations occur more slowly.

Examples of DNA viruses such as smallpox. These viruses spread through human populations and were often fatal. But once vaccinations were developed viruses like smallpox were contained and all but eradicated. RNA viruses, on the other hand, replicate without a similar proofreading activity, and as a result, errors in the genetic coding occur. Its these errors that allow RNA viruses, such as influenza and HIV, to mutate rapidly from host cell to host cell, and make it difficult for vaccines and natural immunities to keep up and prepare for new strains of the virus.

How does a virus travel from an animal like a pig to a human?

Viruses spread from one animal to another through close contact, in whatever manner it normally spreads, such as coughing or sneezing in the case of a respiratory virus. Normally, these infections have no impact on the new host since they were not built to infect them. But when one host is infected by two or more strains of a virus like influenza, new combinations can result.

Influenza, for example, has eight distinct segments to its genome, increasing its ability to form new combinations that can include elements of avian flu, swine flu and human flu. It's these recombined versions of the flu that have the potential to cross over and actually spread through a new host.

Pigs are a particularly good incubator as they have receptors for influenza from all three species, said University of Guelph agriculture professor David Waltner-Toews, author of The Chickens Fight Back: Pandemic Panics and Deadly Diseases that Jump from Animals to People.

"So, if they happen to be around people or birds that have influenza, they will pick them up, and the viruses will mix up inside them."

And as the flu spreads, its list of available hosts spreads as well. Until five years ago, for example, dogs were not susceptible to influenza, said Dr. Earl Brown, a professor of microbiology at the University of Ottawa Faculty of Medicine. But the flu has since spread to canines through horses.

What role does the environment play in mutation?

One of the big factors in the mutation rate of viruses is population density, said Brown.

"When you have high density conditions and overcrowding, like you would see in a pig farm, then the mutation occurs much more quickly as it passes from one snout to the next," he said. The kind of virus likely to thrive is also a function of its environment, he said.

A virus that quickly kills its host as it spreads is more likely to thrive in densely populated areas where it can out-compete other viruses, but would die out when the supply of new hosts is in short supply, he said. Conversely, a virus that incubates in the host for weeks and spreads slowly is more likely to thrive in animals like migratory birds, he said.

How much do farming practices contribute?

Human populations have grown over the past few centuries, and in recent decades, the demand for pork and chicken has soared. That has led to the proliferation of large, dense farms with thousands of animals. Waltner-Toews said those result in a number of factors that boost transmission of viruses such as influenza:

• Many genetically similar animals are kept in one place, and their similarity leaves them susceptible to the same diseases.
• The stress of crowded conditions increases the chance that infected animals will show disease symptoms that help the disease spread, such as coughing and sneezing.
• The animals are shipped all over the world.
• People are traveling all over the world, including migrant farm workers brought in from other countries as cheap agricultural labour.

Waltner-Toews suggested that two changes would reduce the spread of new strains of influenza:

• Having larger number of smaller farms instead of a small number of large farms.
• Reducing the reliance of agriculture on global trade.

With files from The Canadian Press