Answer:
The loss of her 'Grandfather and 'Polly' was the wound that Mattie's heart suuffered during the epidemic.
Explanation:
"Fever 1793" is a historical fiction penned by Laurie Halse Anderson. The novel is based on the real event of 'Yellow Fever' epidemic that broke in Latin America and Africa.
Matilda Cook, the protagonist, is a fourteen-year-old girl, called as Mattie in the novel. The novel narrates the story of the life of Mattie and how this epidemic turned her life upside down.
When Mattie states that 'though they were all healed from the fever but some wounds inside the heart remained that would mend slowly', she was referring to the emotional wounds of loss of loved ones in family. Mattie lost her Grandfather and Polly to 'Yellow Fever', who were very dear to her. And not only did Mattie lost but many others who lost their loved ones to the Yellow Fever were wounded inside their heart. To this wound, Mattie is saying that it would take time to mend.
