The disease that concerned Dr. Dowell was avian influenza, or bird flu. This virus usually strikes poultry, like chickens, ducks, and turkeys. It’s rare for bird flu to make people sick. What is most likely the author’s purpose in this passage?

to explain what the case study is about
to ask a question that he will answer later
to add supplemental information that explains the situation
to help the reader draw a conclusion based on the evidence