Many Zootonic Pathogens Are Highly Virulent, Can Result in High

Many zootonic pathogens are highly virulent, can result in high death rates, and can cause world-wide pandemics. One example, the H5N1 avian flu, not only killed millions of domestic poultry, but also caused the deaths of people who came in contact with sick birds. What is the most probable hypothesis or reason as to why the H5N1 flu can be so deadly to people?

A) This particular pathogen crossed the species barrier and the high virulence can be seen as a side effect of a new pathogen-host association.
B) The pathogen was quick to infect healthy birds.
C) The avian flu would have eventually been controlled; we have to be patient because medicine is close to eliminating infectious diseases.
D) If the H5N1 flu had been half avian and half mammalian, people would have had some immunity to it.
E) Two of the above statements are correct.