A component of Starfleet transporters that scans the matter stream during transport and removes known bacteria, viruses and other harmful agents before a person is rematerialised.
The biofilter works automatically during every transport, comparing what it finds in the pattern against a library of dangerous organisms and stripping them out. It is the reason landing parties in the 24th century rarely go through a decontamination routine.
Its limits drive several stories. It can only catch what it recognises, so unknown infections get through, and it can be adapted for other tasks. In The Next Generation episode Unnatural Selection, the transporter is reprogrammed with a sample of Dr Pulaski's DNA to undo her rapid ageing.
In Realm of Fear, the biofilter is used on Reginald Barclay while he is held in the beam, separating the strange organisms he has seen in the matter stream, which prove to be the trapped crew of the USS Yosemite.