A 20-year-old woman presents with headache, fever, and neck stiffness. On examination, her blood pressure is 100/70 mm Hg, pulse 100/min, temperature 38.6, and the neck is stiff and painful to flex and extend. The ears, throat, and sinuses are normal, there are no focal neurologic signs, and the remaining examination is normal. There are no reported similar cases in the community. Which of the following is the most likely source of her infection?