Very low concentration of a bacterium or virus, at a time when the symptoms of the disease are not yet visible, can be detected by

Polymerase Chain Reaction (PCR) is used to synthesise multiple copies of the gene (or DNA) of interest in vitro using two sets of primers and the enzyme DNA polymerase. The concentration a bacterium or virus is very low, its DNA can be amplified by PCR for early diagnosis. For example, RT-PCR is used for the diagnosis of HIV. Realtime RT-PCR is used for the diagnosis of Covid-19 virus, SARS-CoV- 2. In normal Reverse Transcription PCR, (RT-PCR) The RNA is reverse transcribed into complementary DNA (cDNA), using reverse transcriptase. The DNA is then amplified using PCR. Radioisotope markers are to detect targeted genetic materials on x-ray film. In real time RT-PCR, fluorescent dyes are used, and the results can almost immediately be seen while the PCR process is still ongoing unlike in the conventional RT-PCR in which results can be seen only at the end of PCR.