What is "Captain Trips"?
"Captain Trips" is merely one of the more prominent nicknames for the virus that ravages humanity in The Stand, with another nickname being "Tubeneck" (named for the final stage of the virus, when the throat area of the victim turns black and swells up, killing the victim and forming a kind of "tube" around the victim's throat). The project that created the virus is called "Project Blue".
Captain Trips is highly lethal, with a 99.4% infection and fatality rate (this figure is given by General Starkey, who was in charge of Project Blue - neither the book nor the movie provide any indication of how he or his project arrived at this figure). Its lethality is derived from its nature as a "shifting-antigen" virus, meaning that as soon as the body's immune system has shifted to a defensive posture to fight off the virus (providing antibodies, etc, as in the case of the normal flu virus), the virus shifts to a different form, and continues to do so until it finally overwhelms the victim's immune system (at which point, the victim enters the final "tubeneck" stage, and dies).
Captain Trips is also highly contagious, spreading from victim to victim in relatively close contact and rapidly spreading out of control (it gets carried across the United States in a matter of hours to days, simply by face-to-face contact). This is particularly because it takes days for the virus to kill the victim after the initial mutation after killing the Project Blue staff, and during that period its symptoms are virtually indistinguishable from normal flu symptoms (which, in the book and movie, ultimately leads to the compromise of the CDC and Stovington medical facilities - in the case of the former, a nurse with flu-like symptoms ignores a warning to report any such symptoms to a supervisor, and infects everyone there).
The conditions upon which its release occurred are never completely specified. The obviously shown reasons amount to a series of mistakes and screw-ups, like Charlie Campion's narrow escape from the base where Project Blue was located. Flagg may have had a role in this, although it is never exactly specified (the movie implies this more than the book).
The book points out that Captain Trips was not the only Shifting-Antigen virus made - others included Super-cholera, Super-Legionaire's Disease, and so forth.