Which is an example of virulence as an evolving property? Choose all that apply

a. The 1918 influenza pandemic primarily killed young adults rather than elderly persons which is more typical of influenza both before and after that time.
b. Historically, smallpox appeared to shift from a highly virulent pathogen which killed its victims quickly to one that did not kill immediately but more easily spread from person to person.
c. As hosts evolve more effective defenses against a given pathogen, the pathogen in turn will evolve virulence factors to combat those defenses.
d. Cold viruses which generate more coughs and sneezes while allowing the host to remain mobile will have an evolutionary advantage over those that cause their host to be bedridden.