"Code Orange" by Caroline B. Cooney. New York : Delacorte Press, 2005.
Mitty, who never exerts himself in school any more than absolutely necessary, becomes intensely and personally interested in his "infectious diseseases" writing assignment. Challenged to find titles about smallpox for his bibliography, he investigates some old books his mother has purchased from a doctor's estate - and discovers an envelope containing 100-year-old smallpox scabs. After handling them and accidentally breathing in some of the old dust, he becomes convinced he has infected himself and all of New York City.