It scans all life forms that go through the are and if an unauthorized life form is detected, it emits a loud alarm and blast wide-range rays which can blind humans and kill any life form whose DNA pattern isn't entered into the computer controls. It can be programmed for individuals such as humans or for a species. Bio Filters could scan Controllers and the Yeerk inside the Controller's head simultaneously. If a supposed host scanned negative for Yeerk infestation, they would be immediately terminated.