An exotic but normally harmless rhinovirus whose presence aboard the USS Lantree provoked the engineered immune systems of the Darwin Station children into producing an antibody that aged ordinary humans to death.
Thelusian flu is a mild viral illness that poses no serious danger to healthy humanoids. Its importance lies in a single case recorded in the Next Generation episode Unnatural Selection.
When the Enterprise-D finds the supply ship USS Lantree adrift with its whole crew dead of apparent old age, the medical log shows only one recent illness: the first officer had been treated for Thelusian flu a few days before the ship visited the Darwin Genetic Research Station on Gagarin IV.
Doctor Pulaski eventually works out the connection. The station's genetically engineered children carry an aggressive immune system that attacks threats at a distance, and on meeting the flu virus it produced an antibody that also rewrote the DNA of ordinary humans, ageing them rapidly. Pulaski herself is infected and is saved only by using the transporter and a sample of her earlier DNA.