Kirk suffering the side effects of the vaccine
Melvaran mud fleas are a delicacy of questionable quality.
In 2152, while imprisoned on an Enolian transport, Zoumas described the taste of Melvaran mud fleas to Commander Trip Tucker in minute detail. According to Zoumas, once one got past their texture, they really don't taste that bad. Furthermore, the fun part was keeping them in one's mouth, as they jump around until one crunches down on them. (ENT: "Canamar")
In 2258 of an alternate reality, Leonard McCoy injected a vaccine to protect against viral infection from Melvaran mud fleas into the neck of James T. Kirk. He did so in order to give Kirk the symptoms so that he could bring him aboard the Enterprise as a patient. (Star Trek)
In the novelization of 2009's Star Trek by Alan Dean Foster, McCoy specifies that the vaccine is derived from an emulsion of the flea's internal organs.