Molecular electronics and nanoscale chemical sensors could enable the construction of microscopic sensors capable of detecting patterns of chemicals as they flow passively in a fluid. Information from a large number of such devices allow the estimation of properties of tiny chemical sources in a macroscopic tissue volume. Although such devices cannot yet be fabricated, estimates of plausible device capabilities in small blood vessels allow the evaluation of their performance for typical chemicals released by tissues in response to localized injury or infection. The devices can readily discriminate a single cell-sized chemical source from the background chemical concentration, providing high resolution sensing in both time and space. By contrast, such a chemical source would be difficult to distinguish from background when diluted throughout the blood volume as obtained with a blood sample. These microscopic, programmable devices could also aid treatments, such as precisely targeting drug delivery and improving speed and accuracy of microsurgery.