Levodian flu was an illness harmless to most humanoids. Its symptoms included fever, chills, and the sniffles.
The Levodian flu virus was easily adapted as a way to spread bioengineered plagues. By the mid 23rd century it had been in use within the Klingon Empire for biological projects for over one hundred years. A Trill colony was once infected by the Levoidan flu virus, causing them to develop facial ridges and lose the spotting that the vast majority of Trill possessed. (TLE novel: Excelsior: Forged in Fire)