Flagellum is a lash-like appendage that protrudes from the cell body of certain bacteria.
Some DNA bacterial viruses use flagella to attach to the host cell. This contact with the flagellum facilitates concentration of phage particles around the receptor on the bacterial cell surface.
Usually these interactions take place via the phage tail. However, caulobacter crescentus phages phiCb13 and phiCbK seem to use their head filament instead.
Salmonella phage Chi for example infects motile strains of enteric bacteria by adsorbing randomly along the length of their flagellar filament and then injecting its DNA into the bacterial cell at the filament base. A phage, iEPS5 even seems to directly inject its DNA into the flagellar filament .
Choi Y, Shin H, Lee JH, Ryu S
Appl Environ Microbiol. 2013 Aug;79(16):4829-37