Please carefully/completely read and answer all part(s) of each SHORT ESSAY question: Virulence is the degree of illness

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Please carefully/completely read and answer all part(s)
of each SHORT ESSAY question:
Virulence is the degree of illness or damage
caused by a pathogen such as a virus or bacterium. Consider
the co-evolution of host and pathogen from the first time an
emerging pathogen is able to infect a new host species (for example
the first time HIV or SARS-CoV-2 ever makes a human sick).
How do you measure fitness of the pathogen? How do you
measure fitness of the host? How do you expect virulence to change
as the host and pathogen co-evolve over hundreds or thousands of
years? Are there any features in the nature of the pathogen
that might lead to either increased or decreased
virulence?