David was Human child born in 2353 as part of a research project that developed aggressive immune systems capable of destroying disease organisms. However, the antibodies of the children were capable of attacking other Humans and hyperaccelerating the aging process, killing them within days because of old age.
David was beamed aboard the USS Enterprise-D in suspended animation in 2365, so Doctor Katherine Pulaski could scan for the disease, unaware at the time that David was a carrier. She became infected with the disease, but was cured by using the bio-filters of the transporter. (TNG: "Unnatural Selection")