Final answer:
The influenza viruses that typically infect humans come from Influenza A, B, and C viruses, with Influenza A being the most pathogenic. These viruses can mutate quickly and are highly contagious, with types like the H7N9 and H5N1 presenting significant risks for pandemics.
Step-by-step explanation:
The influenza viruses that infect humans are primarily derived from types Influenza A, Influenza B, and Influenza C viruses. Influenza A viruses, which can infect a variety of animals including pigs and birds, are the most virulent and are responsible for seasonal pandemics each year. These viruses have the ability to mutate via processes known as antigenic drift and antigenic shift. Influenza B virus is less virulent and is sometimes associated with epidemic outbreaks, but lacks significant animal reservoirs. The Influenza C virus typically produces the mildest symptoms and is not commonly linked to epidemics. The potential for an influenza virus to cause a pandemic was illustrated in 2013 when a new strain of H7N9 influenza arose in China, which had a 33% mortality rate but fortunately did not spread easily from person to person. Viruses like the avian influenza virus can cause zoonoses, being transmitted from animal hosts, such as birds, to humans. The World Health Organization and the Centers for Disease Control and Prevention keep a vigilant eye on influenza outbreaks to prevent such threats from turning into worldwide pandemics.