Normally, an influenza virus is just called an influenza virus – or "the flu." But now and then we'll hear about avian influenza ("bird flu") or swine flu (also called influenza A H1N1). But they affect people. So why are they named after animals? Do we need to be worried about eating cooked pork? (No, we don't.) In fact, it's more technical: The reason for these names it has to do with what is causing the specific flu and what that flu-causing virus is "made" of.

The basics: What is the flu anyway?

Any time there is an infection, there is something causing that infection. The person or animal that is sick is called the host. In the case of flu, the thing causing the infection is a virus – an influenza virus, of which there are many types and strains (subtypes). The virus is made up of genetic material. These very basic bits of biological programming material need to replicate within a living cell, the host, to multiply and spread.

Flu viruses are like freight trains?

Imagine a freight train with many boxcars. You now have a basic mental picture of a virus. The virus is made of many pieces of genetic material, usually either RNA or DNA, strung together like boxcars on a train. All of these pieces of genetic material contain information that tells the virus what to do within a host and, specifically, what to do within the host's cells.

When a virus enters a host, it wants to multiply, make more of itself, and thus infect other cells. Most of the time, this multiplying, also called replication, takes place with no problems and the new viruses are exact copies of the original virus – like a new train with the same boxcars in the same order. But sometimes, during the replication process, there's a small mix-up: perhaps one or more pieces of genetic information are deleted or copied more than once. It's like making another copy of the freight train, except that a certain boxcar is put in a different position or a duplicate boxcar is added, making it slightly different from the original freight train. This is referred to as mutation. New freight train, new virus – you get the idea!

Bird flu vs. swine flu vs. any flu

Normally, the viruses that affect one species (e.g., birds) don't necessarily affect another species (e.g., humans). A virus needs the right kind of receptor on the surface of a cell to attach itself to (think North American versus European electric plugs). Human strains of the influenza virus tend to attach to receptors on cells we have in our upper respiratory tracts, while avian (bird) influenza viruses attach to receptors on cells specific to bird species. But some animals – notably pigs – have cells with receptors that both human and avian viruses can attach to.

Pigs are considered a "mixing bowl" for avian and human influenza viruses because they can be infected by both types of influenza. When a pig cell is simultaneously infected by a human flu strain and an avian flu strain, replication may occur in which pieces of genetic material – the "boxcars" – can mix and match, producing a new strain of virus that can infect more than one type of host. In other words, if an avian influenza virus picks up a piece of genetic information from a human influenza virus, it helps that newly reassorted virus enter human cells, causing infection.

Why are new strains a cause for such concern?

The flu that circulates every year is a major public health concern. The Canadian Public Health agency reports that the flu and complications related to the flu account for about 3,500 Canadian deaths every year. That's why it is so important to get a flu shot to protect you from this virus. And because of the mutations that can occur during replication, the flu virus changes often, so you need a flu shot each year to protect against the new virus strain.

When strains of the influenza virus reassort with pieces of genetic information from a combination of species into a new strain, the public health concern grows, as science and the medical world do not know what symptoms and what severity of flu that new strain will produce in humans.

Once a new strain emerges, science races to find out what strains combined to form this new arrangement. It also looks at how it is behaving within the human population to get an idea of its mildness or severity (also called virulence) and to decide what steps to take next.

Gisèle Kane