Where does the Influenza Virus come from and What causes the Flu?

Although you might not think of the flu as a killer disease, an average of 20,000 Americans die of it every year, that’s about four times as many people as die from the diarrheal illnesses caused by eating contaminated food.

Yet many people who are sure they are going to suffer an agonizing death from eating potato salad contaminated with salmonella view the flu as a sort of mild and unavoidable annual unpleasantness.

Most of the people who die from the flu are older than 65 or younger than 1 year. In the ages in between, deaths usually occur from pneumonia (either primary viral pneumonia or bacterial pneumonia as one of the complications of influenza infection) and from exacerbations of cardiopulmonary conditions or other chronic illnesses. There are an estimated 114,000 hospitalizations caused by flu infection yearly, and 43 percent of them are people younger than 65.

The flu is caused by two different kinds of virus called influenza A and influenza B. There are numerous subtypes of influenza A, but influenza B viruses are not categorized into subtypes. These viruses have RNA rather than DNA as their genetic material.

When an RNA virus reproduces, it doesn’t make an exact copy of itself like the DNA cells in our bodies do. Instead, “errors” are constantly being introduced. Poliovirus is another example of an RNA virus, but, fortunately, it’s an exception to the general rule. It has only three different types, and that’s one reason why the vaccine against it has been so successful. Once the polio vaccine gives us immunity, we are immune to polio. Not so with the flu.

There are two kinds of changes that occur in influenza A viruses. The first is a series of mutations that happen over time, causing a gradual change in the virus that makes it able to avoid your immune defenses so that you’re susceptible throughout your life. This gradual change is called antigenic drift, and it gives the virus great staying power, a vaccine effective against one generation is no longer effective against the next.

Because of this, new vaccines are needed every year to combat “this year’s strain” of flu virus. The other kind of change is more abrupt and involves changes in the hemagglutinin and neuraminidase surface proteins of the virus, the proteins that allow the virus to infect a cell, and to which the body’s immune system reacts. This change, called antigenic shift, suddenly produces a new subtype. While type A viruses can undergo both kinds of changes, type B viruses only demonstrate the more gradual antigenic drift.

These rapid changes in the genetic material of flu viruses allow them to infect animals as well, and the virus may be of animal origin in the first place.

Some of us remember the swine flu scare of the late 1970s, which turned out to be less of a threat than first thought. This flu virus developed from pigs that were infected simultaneously with a pig and a human form of influenza A. The two viruses exchanged genetic information in the pigs, creating an entirely new strain that was infectious to humans. In 1997, an influenza outbreak began killing chickens in markets in Hong Kong. The virus was a type of influenza A called H5N1.

An “H5N1” influenza virus means that its surface proteins are hemagglutinin type 5 and neuraminidase type 1. The government of Hong Kong immediately closed all retail and wholesale markets selling chickens and destroyed more than 1 million infected birds. The vigilance of the Hong Kong government was fully warranted: 18 people in Hong Kong were in fact infected, and six of them died from complications of the disease.

Destroying the chickens seemed to banish the virus, but not permanently. The bird flu returned twice in 2001, with the government ordering the destruction of chickens, which had come from China, both times.

Researchers now believe that a change in one particular gene of the virus that is used in viral replication is what allowed the disease to move from birds to humans. Apparently pigs are not necessary as an intermediary in the development of pandemic strains of the flu.