eli5: how do medical labs find out which bacteria or viruses are in a sample if they do not know what to look for?

Question

So I do not mean a covid test where it is clear what pathogen to look for. More like:
“Hey lab, here is a sample of a patient that feels bad. Please find out what pathogen is inside of it.”

How does it work? And how does it work so fast that they usually return the results within 1 or 2 days?

In: 6

Answer 1

Thats why the doctors tell them a few types that make sense with the symptoms the patient shows.
Once you know it should be one out of x pathogens you make a PCR test against this group and see if you got a hit.