  Infant deaths and old age deaths are very different. The former are mostly
due to severe congenital malformations of one or a small number of specific
organs. On the contrary, old age deaths are largely the outcome of a long
process of deterioration which starts in the 20s and affects almost all organs.
In terms of age-specific death rates, there is also a clear distinction: the
infant death rate falls off with age, whereas the adult and old age death rate
increases exponentially with age in conformity with Gompertz's law.
  Clearly, it would be satisfactory to explain the two phenomena as being two
variants within the same explanatory framework. In other words, a mechanism
providing a combined explanation for the two forms of mortality would be
welcome. This is the purpose of the present paper.
  We show here that the same biological effects can account for the two cases
provided there is a difference in their severity: death triggered by isolated
lethal anomalies in one case and widespread wear-out anomalies in the second.
We show that quite generally this mechanism leads indeed, respectively, to a
declining and an upgoing death rate. Moreover, this theoretical framework leads
to the conjecture that the severity of the death effects, whether in infancy or
old age, are higher for organisms which are comprised of a larger number of
organs. Finally, let us observe that the main focus of the paper is the drastic
difference of the age-specific death rates (i.e. decreasing versus increasing)
because this difference is found in many species, whereas the question of the
best fit (e.g. Gompertz versus Weibull) is rather specific to human mortality.

Biological Physics