Hello
Pyrexia of unknown Origin or P U O may be due to these reasons, these include.
1
Typhoid fever , if duration of fever is long i.e.>10 days with morning rise of mild grade fever with mild chills are the characteristic feature of typhoid fever. Diagnosis can be confirmed by blood examination for
Widal test and serology test.
2
Tuberculosis , if fever is > 3-4 weeks with evening rise of temperature usually associated with cough . Diagnosis can be confirmed by clinical history , X ray chest and tuberculin test.
3
Jaundice , as this is one of the most important cause of fever , so get in blood examination for serum
bilirubin, S G OT , S G P T ( liver function tests).
These above mentioned causes are most common causes of P U O in India . Besides there are also so many causes , dengue , malaria etc.
Since Kolkata is a big city and you can consult PHYSICIAN and get his opinion and deal as mentioned above.
Good luck.