Colorimetric and fluorescent assays and biosensors detect viruses, bacteria, parasites, and other pathogens. They consist of biologically or chemically specific receptor ligands attached to a polydiacetylene backbone. Ligands may include nucleic acids, proteins, antibodies, carbohydrates, gangliosides, peptides, enzymes, or small molecules. The membranes change color when disrupted, and the intensity of the color change allows quantification of the analyte's concentration. Biosensors allow direct detection with reasonably small amounts of analyte, and unlike immunoassays, they can provide specific detection with ligands other than antibodies.