Detection of moving objects in videos is a crucial step towards successful surveillance and monitoring applications. A key component for such tasks is usually called background subtraction and tries to extract regions of interest from the image background for further processing or action. For this reason, its accuracy and its real-time performance is of great significance. Although, effective background subtraction methods have been proposed, only a few of them take into consideration the special characteristics of infrared imagery. In this work, we propose a novel background subtraction scheme, which models the thermal responses of each pixel as a mixture of Gaussians with unknown number of components. Following a Bayesian approach, our method automatically estimates the mixture structure, while simultaneously it avoids over/under fitting. The pixel density estimate is followed by an efficient and highly accurate updating mechanism, which permits our system to be automatically adapted to dynamically changing operation conditions.