For RNA viruses like measles, a synthetic second strand is created to form cDNA for amplification.

“and in in the case of measles virus you have a you have something a preliminary step they're called reverse transcription where where because the virus is a RNA is a single strand rather than DNA which is a double strand RNA ribonucleic acid is a single strand so for this this technology to work you need two strands  for reasons which I'm sure you'll probably get to  so you then so what happens initially is a a second strand is created  synthetically so that you then have what is called C DNA which is a which is is a kind of an artificial construct of DNA”