Crusade is a spinoff TV show from J. Michael Straczynski's Babylon 5. It occurs several years after the events in Babylon 5, just after the movie A Call To Arms.
A race called the Drakh have released a nanovirus plague on Earth, which will destroy all life on Earth within five years if it is not stopped. To that end, the long-range explorer ship Excalibur has been sent out to look for anything that could help the search for a cure.
As he had with Babylon 5, Stracynski planned a five-year storyline for Crusade. Ongoing disagreements with the network, however, meant that only thirteen episodes were ever made.