Alien parasites carrying a viral infection; in the 2009 film Leonard McCoy injects James Kirk with the vaccine so that its dramatic side effects will justify bringing him aboard the Enterprise.
Melvaran mud fleas are known only as the source of a viral infection against which Starfleet Medical keeps a vaccine. The vaccine itself makes a healthy person look alarmingly ill for a short time.
In the film Star Trek, released in 2009, cadet James Kirk is grounded pending a hearing over the Kobayashi Maru test when the fleet is ordered to Vulcan. His friend McCoy cannot bear to leave him behind, jabs him in the neck with the vaccine in the shuttle hangar and then insists to the duty officer that medical regulations allow him to bring a patient in his care aboard his assigned ship.
The ruse works, but Kirk pays for it. He loses the vision in one eye, sweats, and then has an allergic reaction that swells his hands to a grotesque size and numbs his tongue, all while he is racing to the bridge to warn Captain Pike that the ship is flying into a Romulan trap.