Influenza, or the flu, is a highly contagious respiratory illness -- one that comprises hundreds of variations of a virus that are constantly mutating. Simple contamination can lead to incredibly virulent and infectious strains of influenza that can spread quickly throughout a population of poultry or other livestock, or can become infectious in humans and reach pandemic levels, such as the outbreak of H1N1 strain of flu in 2009.

How Do Flu Viruses Change?

Flu virus strains are constantly changing and mutating, with changes that can be slow and gradual or sudden. There are two main types of changes that can be of concern to humans:

●     Antigenic drift, which refers to changes in the flu virus that happen slowly over time, causing changes in the seasonal flu that make the virus different enough for our bodies to be unable recognize the virus from year to year. This is why we need to get a new flu vaccine each year.

●     Antigenic shift refers to changes that happen suddenly. This occurs when two different strains of flu infect the same cell and combine, potentially creating a new flu subtype and allowing the transmission of the virus from animals to humans. Because people have little or no immunity to new flu subtypes, these new strains of influenza have the potential to become severe flu epidemics or pandemics.

How Does a Flu Strain Become Infectious to Humans?

Flu strains become contagious to humans because of antigenic shift. These flu strains either will pass from one animal to a secondary type of animal, such as a chicken or pig, and contact with humans will expose this livestock to human flu strains that genetically combine with the new strain to make it contagious. It also may mutate on its own and make an immediate jump into the human population.