Biofilter

From Memory Alpha, the free Star Trek reference.

A biofilter is a scanning device that analyzes an incoming transporter matter stream for known biological anomalies. When it detects such anomalies, like viruses, it will try to remove them from the stream. The transporter operator will always be warned when the biofilter can or cannot remove biological anomalies. The exception is that when microbes exist as both matter and energy, the biofilter cannot filter them out because it cannot distinguish them from matter stream.

Biofilters are also used in replicators like those on Deep Space 9. They were unable to filter out the aphasia virus developed by Dekon Elig and Surmak Ren. (DS9: "Babel")

After Deanna Troi, Miles O'Brien and Data were taken over by Ux-Mal criminals, the biofilter scans showed that their nervous systems were generating high levels of synaptic and anionic energy. (TNG: "Power Play")

The biofilter can be reprogrammed to filter out specific molecular patterns. (TNG: "Realm of Fear")