This interrupted case study tells the story of “Callie,” a 26-year-old baker from San Francisco who runs a successful new cupcake business with her fiancé, Jeremey. For some time Callie has not felt well, experiencing fatigue, difficulty breathing, stress, and a persistent cough. When at last she visits her physician she is diagnosed with an active tuberculosis (TB) infection. The story provides a context for students to learn about TB infection, including how it impacts the lungs and gas exchange in the alveoli, and how one might contract and treat TB. Students will compare Callie’s symptoms to a list of known respiratory diseases and disorders to narrow down possible causes, walk through a visit with a physician, review blood test results, compare and contrast active vs. latent TB infection, and conclude by applying what they’ve learned about TB to gas exchange in the lung. The case is especially appropriate for use in an introductory anatomy, microbiology, pathophysiology, or general biology course.