Bad ending: Plague

The first bad ending i've seen in Babylon5.
This race of aliens was living on Babylon 5 but all of a sudden they started to die, one after the other. After investigating, Dr Franklin (head physician) found out tht there was a virus about the ship killing only that kind of alien. Turned out the aliens knew all along, but because they were afraid to appear like they had 'upset the gods', they did not mention it. They believed the virus was the result of impureness among their kind and that to stop it, they must pray and repent together.
Ignoring Franklin, who suggested they had to find a cure, a way to slow the spread, and if the virus could affect other races, etc, the alien incharge of the others had them gathered together in a quarantine zone, which the doctor knew would only spread the virus faster. So while all the aliens sealed themselves in that one area, the medical team and the alien leader started trying to find a cure.
Believing that help should be given when needed, Minbaris Delem and Lennier both decided to enter the quarantine zone to help where they could and give the aliens a sense of hope. John Sheriden was pissed of cause. He liked Delem, and letting them into the quarantine zone exposed them to the virus which could kill them if it jump species, and meant that he could not let them out once they entered.
Delem and Lennier didn't care, though they were both saddened by the chance they may not leave. They were dedicated to giving aid, and entered anyway. While there, they went about helping the sick and tried to give them hope and support.
Some time later, when Franklin discovered a cure (by then the alien leader had died as well)that could help slow down the deaths caused by the virus, John, Susan, head of security Garibaldi and himself entered the quarantine zone to administer the cure and save the aliens.
When they entered, they found Delem and Lennier huddled together amongst all the dead aliens, over 5000 of them.
Both Delem and Lennier had watched all of them die of the virus, and both of them were shaken and close to tears. Lenneir helped Delem up and they staggered towards the entrance past all the dead, Lennier leaving first. Seeing John, Delem started crying and held onto him while the rest of them looked on at all the dead aliens, Susan holding the late vaccine in her hands.
Later a news report was shown, telling of how the virus had spread on the alien's homeworld and killed all of them, including those on nearby colonies, etc.
The whole species was wiped out by one virus and the chances there were any of the species left were as good as nothing.