By the time the pathogens has produced a symptom conc of them very high.why so

Hi Nihala,
When we get symptom it means the pathogen (microbes/disease organisms) fighted with immune cells (WBC) and defeated them.
Once the immune system is defeated then it starts increasing its population,
starts consuming food from the host and starts producing the symptom.
As you know the symptom is a sign of disease formation and increase in the disease.
After the occurence of symptom the disease is in an detecable level (ready to diagnose).