Final answer:
A flu virus can radically alter its structure through mutation and processes such as antigenic shift, which change surface proteins like neuraminidase and hemagglutinin. These mutations help the virus evade previous immune responses and vaccines, leading to the need for new vaccines every flu season. Close monitoring of these emerging viruses in environments where interspecies transmission can occur is crucial to prevent epidemics.
Step-by-step explanation:
A flu virus can radically alter its structure through mechanisms like mutation and antigenic shift. These alterations occur at the viral surface molecules, particularly the proteins neuraminidase (N) and hemagglutinin (H). Mutations in these proteins happen frequently and can sufficiently change the virus such that it evades last year's immune responses and vaccines, necessitating new vaccines each season.
The structure of influenza viruses includes these surface proteins, which function in attachment to the cells (H) and detachment after the new virions are released (N), are points of antigenic variation. To create a global epidemic, the virus must have a suitable host in which to replicate and mutate, producing novel strains that are not recognized by pre-existing immune systems.
Viral reproduction is a rapid process that introduces errors, leading to changes in surface proteins, allowing the virus to infect new hosts. This evolutionary process, especially in environments where animals and humans closely interact, such as on poultry and swine farms, can give rise to new strains that spread quickly. Scientists monitor emerging viruses heavily, aiming to pre-empt potential epidemics.