DX is a prion disease featured in The Lost World and mentioned frequently in tie-in material and merchandise for the film adapation of the novel. Prions are the simplest disease-causing entities known, even simpler than viruses. They are protein fragments. They are so simple they can't even invade a body. They have to be passively ingested.
In The Lost World
It was theorized by Ian Malcolm that the dinosaurs on Isla Sorna developed a prion disease called DX, from a bad batch of sheep protein extract. The dinosaurs were being fed goat's milk, which is very hypoallergenic; the problem came from the handlers using ground-up sheep meat that were infected with prions to feed the carnivores. The Compys were scavengers and used on the island as waste management by consuming the feces of other dinosaurs, spread the disease over carcasses which would infect other dinosaurs. Eventually, all the Raptors were infected; a single bite from an infected carnivore can infect another dinosaur which would eventually lead to the entire island being infected with the disease, ensuring the re-extinction of the dinosaurs.