Can a disease be detected before its symptoms appear? Explain the principle involved.

(N/A) Yes,a disease can be detected before its symptoms appear.
Normally,the presence of a pathogen (bacteria,viruses,etc.) is suspected only when it has produced disease symptoms. By this time,the concentration of the pathogen is already very high in the body.
However,a very low concentration of a bacteria or virus can be detected even when symptoms are not yet visible.
The principle involved is the amplification of their nucleic acid ($DNA$ or $RNA$) using $PCR$ (Polymerase Chain Reaction).
$PCR$ creates millions of copies of the specific $DNA$ sequence of the pathogen,making it detectable even if the initial sample contains only a few copies.