You are doing a rotation in the hospital's clinical laboratory. A sample of cloudy cerebrospinal fluid (CSF) from a suspected meningitis case arrives and you are told to gram stain it, and then to plate it on blood agar and chocolate agar. In the Gram stain you find gram- rods of varying size and shape. You also find a lot of bacteria inside phagocytic cells. They are not diplococci. Colonies grow on both of the plates you inoculated. Later, the charge nurse tells you that the patient, a 3-year-old girl, has not received any childhood vaccinations. What is the most likely causative organism? Why?