Current programs usually scan for fragments of code that identify particular, known viruses, and eradicate them when found.

ECONOMIST: A thousand ills require a thousand cures

There, a DNA-sequencing machine fired lasers at the fragments and read their genetic code, yielding a computer file in the form of As, Cs, Gs and Ts.

WSJ: Scientists Store King Speech, Shakespeare Sonnets in DNA

When the card is inserted into a reader, the chip checks that the fragments are present and undamaged, and if so returns an identification code.

ECONOMIST: MONITOR