The Darwin Genetic Research Station on Gagarin IV had developed Human children who had aggressive immune systems, capable of attacking disease organisms before they entered a Human body. The children's antibodies sought out and attacked Humans infected with the Thelusian flu, a fact not discovered until 2365, when the entire crew of the USS Lantree was killed after exposure to the children.
In mid-2365, the first officer of the Lantree suffered a case of Thelusian flu. Shortly thereafter the Lantree stopped at Darwin Station.
The first officer transmitted the disease to the rest of the Lantree crew, who began aging at a rapid rate, and later died. The station personnel also contracted the disease, as did Doctor Pulaski later, until a cure was discovered, using the transporter to revert a person's genetic pattern to a previous transporter trace or DNA code. (TNG: "Unnatural Selection")