(EDIT: I don’t know why, but TA isn’t updating to show that I’ve unlocked this achievement.)
(EDIT #2: Apparently the dead Apatosaurus may or may not be a universal occurrence, so take with a grain of salt.)
On Nublar North, once you acquire the vaccine for the dinosaurs’ respiratory malaise (volcanic silicosis), you’ll be tasked with vaccinating every dinosaur on the island to ready them for evacuation.
Very straightforward, just control a Ranger Jeep and drive around the island, medicating every dinosaur you see.
I will say, I had a legitimate scare when one of my apatosaurs died from the illness because I couldn’t get to them in time. (I had a breakout to deal with earlier on, which may have cut my time down.) But after slogging through the mission, I ended up getting the achievement anyway, which led me to one of two realizations:
1) Dead dinosaurs do not count against you; the only requirement is that you vaccinate them YOURSELF.
2) The achievement is buggy.