How does the author use imagery in the novel, The Valley of Amazement?

Imagery (The Spanish Flu):

“He inserted a bulb syringe into one of Edward’s nostrils. When the doctor looked at what he had withdrawn, his expression changed to dark puzzlement. It was thick and tinged with blood. He reassured me that it was not serious. Edward coughed up sputum. It, too, had streaks of red. The doctor babbled on as Edward coughed violently and tried to catch his breath.”