From the official site:

LOCKDOWN
10pm 09/19/02
DOCTORS FEAR OUTBREAK OF DEADLY DISEASE IN SEASON FINALE - In the season finale, fear grips Chicago when Carter (Noah Wyle) and Abby (Maura Tierney) treat two children with ominous symptoms that force the E.R. into quarantine until federal disease experts arrive. As Carter fights to save the youths, the staff is trapped with the suffering patients and a busload of injured and ornery senior citizens. Media swarm the hospital and riot police guard the exits while the restless victims begin a small riot in an attempt to escape the dangerous and stifling conditions inside. Meanwhile, Drs. Pratt (Mekhi Phifer) and Chen (Ming-Na) -- who may have contracted the disease -- put aside their own symptoms as they labor to save the life of frequent homeless visitor Stan the Can (guest star Harrison Page). Laura Innes, Goran Visnjic, Paul McCrane, Sherry Stringfield and Sharif Atkins also star