The Klingon augment virus, a modified version of the Levodian flu
The Levodian flu was a virus apparently common in Federation member planets. It usually ran for a maximum of 29 hours, with the only signs being similar to the traditional Earth flu.
In 2154, a Klingon medical team led by Antaak recovered several examples of DNA from Human Augments originally engineered by Arik Soong. This team attempted to use the DNA to create Klingon Augments with disastrous results. While the incorporation of this DNA did enhance the strength and stamina of the subjects, it produced unwanted side-effects and was ultimately fatal. One of the original experimental subjects suffered from Levodian flu at the time of the experiment. The virus somehow incorporated the Augment DNA and carried it to other Klingons. Antaak believed the Augment DNA changed the virus to propagate itself; he described this modified Levodian flu as the worst threat to face the Klingon Empire since the Hur'q. (ENT: "Affliction")
The Doctor sick with simulated Levodian flu
The Doctor programmed himself with a simulated version of the Levodian flu in 2372 at Kes' request to make him more sympathetic to his patients; The Doctor went along with it to try and show why a "little" illness shouldn't stop anyone working. However, Kes secretly changed the program to make the illness run a little longer. About thirty or thirty-one hours in, The Doctor was uncomfortable, not knowing what was happening to him or why – in that, Kes felt she had made her point. (VOY: "Tattoo")