How does Jim Murphy use imagery in An American Plague: The True and Terrifying Story of the Yellow Fever Epidemic of 1793?

Imagery:

"No one knew that a killer was already moving through their streets with them, an invisible stalker that would go house to house until it had touched everyone, rich or poor, in some terrible way."

"Putting the name yellow fever to the illness was not to be done lightly. Yellow fever was one of the most vicious diseases in the world and could create panic anywhere."