"He'll come out on his own! Eventually."
The Regulan bloodworm was a creature native to Regulus II, a planet in the Alpha Leonis system. It had been described as soft and spineless. Medically, and not unlike the Terran leech, it was quite useful for cleansing a being's lymphatic system. Bloodworms could be introduced into a person's body through their eye. (ENT: "Stratagem", "Doctor's Orders")
In 2151, when suffering from inadequate hibernation, Doctor Phlox suggested the use of Regulan bloodworms to filter a toxic antigenic compound in Ensign Travis Mayweather's bloodstream. Believing that fresh bloodworms would be more effective than ones from Enterprise's sickbay, Phlox contacted the starship's bridge and gave the order to travel to Regulus at maximum warp. The doctor eventually treated Mayweather with the proper medication. (ENT: "Two Days and Two Nights")
In 2152, after Lieutenant Malcolm Reed suffered a wound to his leg, Doctor Phlox treated him with a number of Regulan bloodworms. Despite this, he wished to use even more, but Reed would not allow it, commenting that he still had not found the last one, though Phlox felt it would come out on its own eventually. (ENT: "Dead Stop")
In 2153, Captain Jonathan Archer ordered Phlox to develop a memory removal serum to use on the Xindi-Primate Degra, in order to erase his short-term memories so the captain could extract information from him about the location of the Xindi weapon. Archer then told Degra that his memory loss was a side effect of being interrogated with Regulan bloodworms. (ENT: "Stratagem")
Korax, a Klingon Defense Force lieutenant and first officer of the IKS Gr'oth, insultingly compared Humans to Regulan bloodworms on Deep Space Station K-7 in a conversation which immediately preceded a barroom brawl in 2268. (TOS: "The Trouble with Tribbles"; DS9: "Trials and Tribble-ations")
The reference to Regulan bloodworms by "Trouble with Tribbles" writer David Gerrold was never followed by an explanation of what they did to get the name "bloodworm", and it was never revealed which of the planets orbiting Regulus the worms originate from; according to the schoolroom display, however, the home planet of the bloodworm is Regulus II.
Gerrold pitched a script ("Blood and Fire") for Star Trek: The Next Generation involving infections of bloodworms carried by helpless victims. The story used bloodworm infection as a metaphor for the 1980s AIDS crisis, describing the same kinds of mistreatment and persecution of carriers. Gerrold related, "What I wanted to do was deal with Regulan bloodworms, because we had mentioned them in 'Trouble with Tribbles.' People were always asking about them, but who knows what a Regulan bloodworm is? [....] I worked out the lifecycle of the bloodworms, that they grow in your blood until they reach a certain point and then, like malaria, they explode and start looking for new flesh." (Lost Voyages of Trek and The Next Generation, pp. 88-89)