The invention involves the detection of virally infected cells by antibodies or antibody fragments which selectively bind to TSG101. TSG101 is on the surface of mammalian cells, and thus available for detection by antibodies, during viral budding—a phenomenon wherein viral particles escape a virally infected cell after propagation in that cell, so as to infect other cells. To achieve budding, a protein, TSG101 is “hijacked” and misdirected to, or mis-expressed on, the surface of the infected cell. Antibodies can be used to selectively detect such infected cells. Certain TSG101 antibodies may provide therapeutic benefit when administered to infected mammals.