Every event taking place anywhere in this universe essentially has a cause it may be apparent in some instances and may not be in some. Similarly, the disease process has a causative factor; this causative factor is Hetu/Nidan.

In Ayurveda the best medicine is that which makes good health and the best physician is one who cures the patient from diseases. Treatment of diseases will be effective only with good knowledge of Nidan (diagnosis). The term Nidan stands for both Vyadhijanak (cause of disease) and Vyadhibodhak (knowledge of disease). Vyadhijanak nidan is called as Hetu and Vyadhibodhak nidan comprises of Nidan, Purvarup, rup, Upashay and Samprapti.

A particular factor can be called as nidan only when it will develop a complete disease process (Setikartavyata) in the body either immediately or after a certain period, because Nidan produces disorder through aggravation of doshas.  Doshas aggravated by respective etiology give rise to disorder.

Synonyms - Etiology (nidan) has many synonyms like nimitta, hetu, karan, yoni, ayatan, pratyay, utthan, karta and mula.

Types - There are following 14 types of Nidanas given in Madhavnidan  madhukosh tika.

Type – 1: Ayurveda has identified three main etiological factors as being responsible for all sorts of diseases and ill health. They are:

                1) Asatmyendriyartha samyoga  
                2) Prajnaparadha
                3) Parimana.

Type – 2  1) Sannikrusta hetu
                2) Viprakrushta hetu
                3) Vyabhichari hetu    
                4) Pradhanika hetu

Type – 3  1) Dosha hetu
                2) Vyadhi hetu
                3) Ubhaya hetu

Type – 4   1) Utpadak hetu 
                 2) Vyanjak hetu

Type – 5   1) Bahya hetu
                 2) Abhyantar hetu.

Role of Hetu in Chikitsa (Treatment)

The chikitsa mainly consist of Nidanparivarjan along with other chikitsa, it is not the complete stoppage of all hetus but it is very important to consume hetus limited as per only need, the excess must cause harm. It is useful to guide about pathyapthya to the patients as hetu becomes apathya in vyadhi.

In difficult cases where diagnosis is elusive, exploratory therapies (Upashaya- Anupshaya) in the form of trial and error method is adopted to arrive at a diagnosis. Exploratory therapy involves the use of medicines, diet and activities as Hetuviparit chikitsa (Antagonist to cause).

If there is any doubt about diagnosis of the disease, observation of the etiological factors leads to confirmation of the same.

In these ways, Nidana is helpful in diagnosis of disease.