How Viruses Evolve
Influenza mutates very fast. Every time the virus replicates, there is an average of about 1.4 mutations in the daughter viruses — permanent changes in the genome. Many of these mutations will be deleterious, causing the mutant virus strain to die out and be replaced by better-adapted versions.
Other changes will have no effect on the daughter viruses' physical characteristics, called its phenotype. This is because more than one combination of bases (the information units of the genome) can code for the same result.
Still other changes will have an effect on the phenotype that is neutral to the virus's abilities. A minor change in the virus's protein coat often has no direct effect on its function. Despite not affecting function, these mutations can be beneficial if they give the virus a new "identity," preventing the host's immune system from recognizing it.
Most rarely of all, a mutation will give a daughter virus a new ability. By chance, once in a great while, a mutation will coincidentally give the virus a protein that enables it to invade a new kind of cell — the cells of a different host species. A disease that can be passed from animals to humans is called a zoonosis, and Influenza A is a zoonotic virus.