Gardnerella vaginalis is a bacterium that leads to the disease called bacterial vaginosis. Bacterial vaginosis is not recognized as an STD but is based on a disturbance of the bacterial balance in the vagina. By disturbing the bacterial balance, a less acidic environment is created in the vagina, which gives certain bacteria, such as Gardnerella vaginalis, the chance to grow excessively. The disease can be caused by keeping a tampon in too long, a certain course of antibiotics and having an IUD fitted.