Bajoran flu is a particularly virulent form of influenza virus that attacks the respiratory system.
During the Cardassian Union's Occupation of Bajor, the flu was rife on the planet, and Kira Nerys saw many of her friends die from the disease as a child. However, even after the occupation the virus was still a problem, but was easily treatable by Federation medicine.
In the year 2370, there was an outbreak of the flu on Deep Space 9, and the entire complement of space station personnel had to be vaccinated against the virus. Unfortunately, the vaccination had an unforseen result on Constable Odo, who became seriously unstable with bouts of memory loss and paranoia. However, the effects soon wore off after a few hours. (DS9 comic: "Lapse")