1. A system for estimating vital signs from passive thermal video comprising: an infrared video receiving module for receiving an infrared video segment of a subject; a contour segmentation module for contour segmenting the video segment to select a region of pixels representing a portion of the subject; an alignment module for aligning the selected region of pixels across a plurality of frames of the video segment; a signal detection module for detecting a signal from the selected region using a thermal wave propagation-based signal detection method; a spatial filtering module for spatial filtering the signal to remove noise not related to the vital sign to be measured; a non-linear temporal filtering module to process the temporal signal corresponding to each aligned pixel sequence; an clustering module for removing outliers of the signal using a pixel clustering algorithm; a frequency selection module for selecting a frequency peak of the signal using a dominant frequency voting technique; and an averaging module for averaging the selected frequency peak to compute an average estimation of at least one vital sign.