Dear Rajeneha,
It is the process of certain organisation to conduct the medical test.
They will be tie up with some hospitals for taking the test. they will under go majorly MRI scan, Blood test, urine/motion culture, ECG etc.
If the person is clearing all the test they certify the person is medically fit for work.
See when medical examination is done each tissue has their own radio waves in MRI scan so if any changes in radio waves signal they will examine the problem ( I am not a doctor but i had studied the process how it works). Certain examples are ear implant, implantable cardioverter defibrillator or pacemaker, or have any metal in their body such as surgical clips. also they know whether the person has been operated in the past. this is how they will find the history.
Only when the person is not clearing the medical test he will not be appointed by the company.