In an indirect immunofluorescence assay (IFA), secondary antibodies are stained with a fluorochrome rather than the primary antibodies. The unstained primary antibodies attach directly to the targeted antigen, and the fluorochrome-attached secondary antibodies then bind to the primary antibodies. Since multiple secondary antibodies can bind to a single primary antibody, this method increases the total number of fluorescent markers attached to the pathogen, making its features easier to visualize against a dark background under a fluorescence microscope.