Also known as: ALL
Nick is a 14-year-old with recurrent leukemia who has undergone radiation and chemotherapy, leaving him immunocompromised with only days to live without a bone marrow transplant from his brother. His treatment causes severe complications including bleeding and GVHD.
Also known as: Fungal infection from chicken feces
Matty, the donor brother, contracted histoplasmosis from dirt contaminated with chicken feces used to build his pitcher's mound. The infection causes fever, enlarged spleen, and critically destroys his bone marrow, threatening his ability to donate and his own survival. The episode's central mystery revolves around diagnosing this infection.
Also known as: Testicular inflammation
Matty develops acute scrotum (testicular swelling) as a symptom manifestation of his systemic infection, initially leading the team to test for specific infections causing this presentation before realizing it's a secondary effect.
Also known as: Heart valve infection
The team initially believes Matty has infectious endocarditis on his mitral valve and considers open-heart surgery to remove the valve, diagnose the infection, and treat the marrow. However, the growth turns out to be fibrous tissue, not infectious.
Also known as: GVHD
Nick develops severe grade 4 graft-versus-host disease after receiving a partial (4 out of 6) bone marrow match when his brother cannot donate. The GVHD causes severe blistering on his legs and feet, excruciating pain unresponsive to steroids, and is ultimately terminal, driving the climactic ethical dilemma.
Also known as: Bone marrow crash
As Matty's histoplasmosis progresses, his bone marrow completely crashes, causing him to bleed from his ears and his blood counts to plummet dangerously. This complication destroys enough marrow that he cannot safely donate to his brother even after diagnosis.