India is a developing country struggling hard to reduce the rate of infant mortality but yet to gain success because it is a country of numerous tribal and non-tribal groups with unique cultures and world-views which affect  the prenatal,  antenatal and post natal developments differentially. The study attempts to highlight the infant morbidity that lead to infant mortality in two linguistically cognate tribes, namely, the Mising of Assam and Minyong of Arunachal Pradesh. Many scholars state that the Mising tribe of Assam have migrated from the hills of Arunachal Pradesh and originally inhabited the same ecology as that of the present Minyongs.  At present; Misings are concentrated in the Brahmaputra riverine areas of Assam and these areas are perennially affected by floods; which is a new stress for them after migration from the hilly terrains. The decadal growth rates have proved that the Mising population has increased and at present they are the second most populated scheduled tribe (plains) of Assam. The number of survivors upto the reproductive age is an important indicator of biological adaptation as well as the outcome of natural selection in the particular stress (flood) and the indices of selection intensity show that the number of survivors is quite more among the Misings than that of the Minyongs. In this point, the number of surviving and healthy infants plays a significant role and its an urgent need to highlight the morbid conditions of these two tribes.