The observation of photoluminescence (PL) activation, enhancement, fluorescence intermittency (FI), and decay over the photochemical lifetimes of single quantum dots (QD) is reported. The ability to count individual QDs along with PL intensities has revealed a measurable nonlinear correlation between the total fluorescence and QD population that is the result of enhancement and decay in the quantum yield (QY) of individual dots. This allows the differentiation between ensemble PL enhancement due to the fluorescence activation of a dark, non-emitting fraction of QDs and that due to genuine QY modification in individual nanocrystals. The evolution of the bright, fluorescent population under continuous illumination is seen to closely follow that of the intermediate in a consecutive elementary reactions scheme. Spectral separation of QD fluorescence trajectories into red and blue components indicates that PL decay is not necessarily associated with blue-shifting of the emission wavelength.