A. Measles generates a weak immune response that allows reinfection of hosts, and therefore does not easily evolve escape variants.

b. Human influenza a virus evolves escape variants, resulting in a distinctive cactus-shaped phylogeny.

c. Measles experiences strong selection for escape variants, resulting in a phylogeny similar to a nonpathogenic species.

d. Human influenza a generates lifelong immunity, so there is no selection for escape variants.

The correct answers are:

b. Human influenza a virus evolves escape variants, resulting in a distinctive cactus-shaped phylogeny.

All types of Human influenza viruses evolve to escape immunity induced by prior infections and vaccinations.

c. Measles experiences strong selection for escape variants, resulting in a phylogeny similar to a nonpathogenic species.

As a result of measles infection, specific antibody and CD4 and CD8 T cell responses are generated and contribute to virus clearance and protection from reinfection.