Theories proposed to explain the disease causation varied from time to time. There was a time when the distribution of food products was not well organized which resulted in the extreme shortage of food; starvation and malnutrition followed. Green revolution was not even heard of. Result was the birth of nutritional theory, putting forth the role of malnutrition as the sole cause of diseases. Later in the beginning of 20th century germ theory was projected as the major etiological factor of diseases. In due course of time, scientists took up environmental and host factors and proposed the theory of epidemiological triad. As time moved on, non-communicable diseases came to the lime light. By the end of 20th century, the above mentioned theories could not explain the causation of diseases, disabilities and death.