Final answer:
Influenza virus is known for its high mutation rate resulting in antigenic variation, which occurs due to antigenic drift and shift. These processes necessitate annual vaccine updates and pose a threat for the emergence of new viral strains.
Step-by-step explanation:
Influenza virus is known for its antigenic variation primarily due to its high mutation rate. This variation is a result of two processes known as antigenic drift and antigenic shift. Antigenic drift involves point mutations leading to minor changes in the virus's spike proteins, whereas antigenic shift is a more dramatic change resulting from gene reassortment when two different influenza viruses infect the same host.
Due to this capacity for mutation, influenza remains a significant threat to human health, prompting the development of new vaccines annually to combat the most prevalent and potentially virulent new strains. The virus's ability to mutate is also a concern for the emergence of new viruses, especially in settings like poultry and swine farms where animals and humans are in close contact.