Shat Kriya Kala or the six significant stages of disease formation in Ayurveda provides the knowledge of how illness originates and manifests step by step. Ayurveda uses these six stages to remove both the signs as well. Each stage provides an opportunity to halt and possibly reverse the spread of the disease. Naturally, the earlier a disease is detected, the easier it will be to neutralize it. 

The concept of Kriyakala, described by Susruta, It is very essential for early diagnosis, prognosis and for adopting timely preventive and curative measures. Out of six kriya kala first three viz., Sancaya, Prakopa and Prasara are subclinical stages while the later three viz., Sthanasamsraya, Yyakti & Bheda are clinical stages of the pathogenesis.

(A) Sanchaya  (Natural accumulation of Doshas) - Sanchaya means accumulation of the dosas in their own natural sites. The dosas in this stages increase quantitatively their quality remaining normal. It is the first stage of Kriyakala with vague and ill defined symptoms along with some features characteristic of the increase of dosa involved. It is characterised by aversion towards similars and attraction towards opposite qualities of diet, drug etc. e,g. vata chaya have symptom of aversion toward dry and cold food.

(B) Prakopa (More accumulation) - It is the stage of excitation of dosas in which the accumulated dosas start getting Prakopa (More accumulation)  qualitatively vitiated and get ready to move from their natural sites being excited by the appropriate aetiological factors.

(C) Prasara (Overflow level) - It is third kriyakala. The term 'prasara' means to spread. It is the third stage of Kriyakala, in which the vitiated dosas spread over and extend to other parts, organs and systems of the body in all directions. Prasara may involve one, two or all the three dosas together alongwith rakta. The dosas in this stage may remain quiscent ormay pass on to sthanasamsraya, depending upon the degree of prakopa.

(D) Sthanasamsraya (Stage of disease augmentation) - This term means getting localised at one place. It is the fourth stage of Kriyakala, in which the excited spreading dosas, having extended to other parts of body, start getting localised at certain sites due to srotovaigunya or a pre-existing defect of the part involved. The week ring of the chain get braked first Localisation of dosas precipipitates dosa-dusyasammurchana (interaction of vitiated dosas with dosyas or local tissues). This is the stage of start of real disease and is associated with appearances of prodromal symptoms i.e.purva rupa.

(E) Vyakti (Stage of symptom manifestation) - It is fifth stage of Kriyakala which characterises by full manifestations of signs and symptoms of the dully developed spacific disease. The vyakti refers to the completion of the sthanasamsraya stage of the disease. It is acute stage of disease.
 
(F) Bheda (Stage of complications) - It is the sixth and last stage of Kriyakala. If the disease is not properly treated at the stage of vyakti, it may become sub-acute, chronic or incurable on account of extensive damage sustained or irreversible structural change having taken place due to neglect of early diagnosis and prompt treatment. The disease in this stage leads to the development of complications and becomes itself the cause for other diseases.

Shat Kriyakala permit the Ayurvedic practitioner to detect and treat illness at the earliest stages, long before permanent damage is done. Even in the later stages, however, it is often still possible for Ayurvedic treatment (Panchakarma)to reverse the course of the disease and provide relief.

Reference:

Sushruta, Sushruta-Samhita along with Ayurveda-Tattvasandipika Hindi Vyakhya by Dr. Ambikadatta Shastri, Part-1, Chaukhambha Sanskrit Sansthan, Varanasi, India, twelfth edition-2001.

Sushrut, Sushrut-Samhita with English translation of text and Dalhana’s commentary along with critical notes Vol.-1 by P.V. Sharma, Chaukhambha Vishvabharati, Varanasi, India, first Edition-1999.