Diagnosis is the systematic method a doctor employs to ascertain from what disease a patient is suffering. The doctor does it by using all his senses ( sight, hearing, touch smell) and microscopic, X-ray and other technical tests.
The experienced, all-around doctor ( general practitioner) relies primarily on the past history of the patient, on the symptoms of his present illness on his examination and on a few routine tests.
Symptoms are of two kinds: objective, those which the doctor can observe and discover, and subjective, those symptoms which the patient discloses or is made to disclose by the doctor.
All disease have some symptoms in common, and that is one reason why a layman will often imagine that he is suffering from a certain serious disease only because a neighbor or relative who succumbed to such an illness also complained of symptoms similar to his own.
That is also a good reason why laymen should refrain from diagnosing and treating disease in serious ailments and when the help of a doctor is obtainable.
On account of this presentation of the same common symptoms by various diseases, even the most experienced physician must often resort to laboratory tests in order to positively differentiate and diagnose a disease correctly.