Multiple realities
(covers information from several alternate timelines)

The Melvaran mud flea was an animal that was edible, but could carry viruses.

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 did not taste that bad. Furthermore, the fun part was keeping them in one's mouth, as they jumped around until one crunched down on them. Finally, unable to bear Zoumas' verbiosity, Tucker sarcastically said he loved "hearing how delicious Melvaran mud fleas are." (ENT: "Canamar")

Melvaran mud flea vaccine side effect

Kirk suffering the side effects of the vaccine

In 2258 of the 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. This circumvented Kirk's academic suspension, which had him remaining on Earth at the time of Nero's attack on Vulcan, as McCoy could bring Kirk aboard at his discretion due to the stipulations of the medical code. (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.