Sarah was exhausted. The long day at work had drained her, and all she wanted was to get home, shower, and collapse into bed. She went to the subway, bought her ticket and stood — waiting for the next train.

When the train stopped with a metallic screech, she stepped inside. The carriage was already half full, but as more people pushed in behind her, it quickly became crowded.

She found a tight spot near the middle, beside a window and wrapped her hand around the cold metal bar. Her body ...