Thelusian flu was an exotic rhinal virus, harmless to Humans.
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")