It would be mistaken, therefore, to assume that the same conditions would prevail in isolated places like North Sentinel Island, where the people are apparently well nourished. So the infant mortality rate there may be above the best found elsewhere, but it is probably lower than many other places.
Is ~400 people a large enough population to support most diseases, or is it possible that they essentially (infectious) disease free by virtue of their tiny population?
I'm also curious what the genetics of a 400-person island look like. I always assumed that the minimum viable population was larger than that -- somewhere in the thousands -- but if the population estimate is correct, and has been for tens of thousands of years, then that's pretty concrete evidence that it's no more than 400.
Several studies, two on the genetics of the Andamanese [1,2] and the other on their linguistics [3] suggest that they have been isolated for 50-70,000 yr. The genetics studies also showed a very low population diversity, so in common parlance they are "inbred". Oddly enough, if they have been isolated for long enough, they will therefore be free of most genetic diseases, since the rare ones will be manifested early on in their isolation due to inbreeding, and then selected against. Genetic conditions that don't affect reproductive fitness could be common, though. Genetic isolation can also result in increased susceptibility to pathogens later introduced from outside said population.
[1]Kashyap, V. K., et al. "Molecular relatedness of the aboriginal groups of Andaman and Nicobar Islands with similar ethnic populations." International Journal of Human Genetics 3 (2003): 5-11.
[2] Thangaraj, Kumarasamy, et al. "Genetic affinities of the Andaman Islanders, a vanishing human population." Current Biology 13.2 (2003): 86-93.
[3]Abbi, Anvita. "Is Great Andamanese genealogically and typologically distinct from Onge and Jarawa?." Language Sciences 31.6 (2009): 791-812.