Disclosed are devices and methods for detecting the presence of a preselected analyte in a fluid sample. The invention provides a device comprising a solid substrate, typically on the order of a few millimeters thick and approximately a 0.2 to 2.0 centimeters square, microfabricated to define a sample inlet port and a mesoscale flow system. A sample is passed through the mesoscale flow system, and the restriction or blockage of flow through the flow system is detected as a positive indication of the presence of the analyte. The mesoscale flow system includes in one embodiment a primary sample flow channel extending from the inlet port, and a fractal region, in fluid communication with the flow channel, comprising bifurcations leading to plural secondary flow channels. The device may be adapted for operation in conjunction with a pump, for example, to induce flow of a sample through the flow system. A detector may also be provided for detecting analyte induced changes in flow properties in the mesoscale flow system. The devices of the invention may be used in a wide range of applications, including the detection of cells or macromolecules, such as viruses.