Nanites are graphene-based nanoscopic (10^-9 meters) robots capable of performing a variety of tasks if programmed accordingly, including making repairs to the human body. Their invention at an unspecified point in the future made 21st century medicine effectively obsolete, as most injuries and diseases could now be cured by injecting tens of thousands of preprogrammed nanites into the patient.
Nanites are produceable using 21st century technology and some Travelers are authorized to carry and use them if necessary. However, they are used sparingly (only when authorized by the Director), and most Traveler teams are expected to have a member trained in 21st century medicine.
Derek (Traveler D-13) provided Grant MacLaren with a dosage of Stage 3 nanites to help him recover from his injuries in the crash of Flight 329. At first MacLaren's injuries were believed to be beyond the nanites' repair capabilities, but he ultimately survived.
Derek appeared again after the shooting at Ellis' farm and secretly injected Grace Day with nanites. After regaining consciousness, Grace shared her nanites with Trevor Holden, ensuring he was not paralyzed.
When Grace and Jenny were both sick with the Faction's weaponized flu virus, Derek offered help once more even though he was not authorized to do so, remarking: "With the Director down, I figure the discretion falls on me." The nanites he offered were not programmed to counteract the virus, but he believed they might buy time if either patient went into organ failure. Grace reprogrammed the nanites to pull the virus into red blood cells where she believed it would be unable to survive. However, the Faction had predicted this countermeasure, and designed the virus to block red blood cells' ability to carry oxygen.
While more effective than any contemporary technology, nanites are not infallible and their usage is not a cure-all. Vincent Ingram claims to have spent his life "being eaten away by a disease that doesn't exist yet," implying that they cannot cure certain diseases. Futhermore, nanites cannot be reactivated after being deactivated.
After MacLaren regained consciousness, Derek advised his patient that he should rest for at least two more hours in order to avoid external scarring, and to refrain from any sexual activity for 36 hours.