Nick was given radiation therapy to destroy his remainign diseased bone marrow and was put in a clean room pending the procedure. Unfortunately, just as the procedure was ready to proceed, Matty started to sneeze and Wilson realized that if Nick got a disease from Matty in the transplant, it would ravage his weakened immune system.
Wilson begged Gregory House to diagnose Matty, even though the condition wasn't life threatening. However, they had to complete the transplant in five days or Nick's compromised immune system would certainly succumb to an infection. House agreed, but his attempts to find a diagnosis only made Matty's condition worse until it was Matty's life that was in danger.
Against House's orders, Eric Foreman went through the bone marrow registry and found a 4/6 match for Nick. However, this transplant resulted in Nick developing graft versus host disease and he was once again near death.
House soon realized the only way to save Matty was to deliberately infect Nick because the disease would progress more quickly in him, allowing them to find a diagnosis that could save Matty. Their parents refused, but Nick volunteered to be exposed.
Luckily, Foreman figured out what was wrong with Matty before Nick was infected. However, Nick was still near death. Foreman convinced Matty to donate bone marrow even though Matty couldn't be given anesthetic. In a very painful procedure that Matty begged Foreman to stop, Foreman harvested enough bone marrow to save Nick as well.