Siddha is a theological term that can apply to a person who has transcended boundaries and is close to divinity, a form of Alchemy (the Siddha Medicine System), Saiva Siddhantham and Yoga with Pranayama traditions and disciplines. It is generic and can come to denote anything as per the context as all are absolutely inter-related.
Vatham, Kapham and Pitham are three humors or thathuvams that are correlated to Vali (air), Azhal (fire) and Aiyam (Water) respectively. These three humors constantly interact with the five primordial elements of the universe or outside to determine the state of body and mind. The ratio between the three has to be constantly maintained at 4:2:1 for a disease-free life filled with health. Any imbalances in the three humors, viz. Vatham, Kapham and Pitham are said to cause diseases or ill-health.
Off late Siddha Medicine system uses some of the modern scientific diagnostic methods like the blood test, scan reports, etc, to find diseases and other medical conditions. But traditional diagnosis methods employed by Siddha still plays a very important role in diagnosis. This diagnosis methodology is notably very unique to the Siddha system which not only identifies the diseases but also their possible causes. The Siddhars have passed on eight diagnostic methodologies and these eight tools are referred to as Envagai Thervu.