Melissa is a 15-year-old high school student. Over the last week, she had been feeling tired and found it difficult to stay awake in class. By the time the weekend had arrived, she developed a sore throat that made it difficult to eat and even drink. Melissa was too tired to get out of bed, and she said her head ached. On Monday morning, her mother took her to her doctor. Upon completing the physical exam, he told Melissa the lymph nodes were enlarged in her neck and she had a fever. He ordered blood tests and told Melissa he thought she had mononucleosis, a viral infection requiring much bed rest.