  After birth setting up an effective immune system is a major challenge for
all living organisms. In this paper we show that this process can be explored
by using the age-specific infant death rate as a kind of sensor. This is made
possible because, as shown by the authors in Berrut et al. (2016), between
birth and a critical age t_c, for all mammals the death rate decreases with age
as an hyperbolic function. For humans t_c is equal to 10 years. At some ages
the hyperbolic fall displays spikes which, it is assumed, correspond to
specific events in the organism's response to exogenous factors. One of these
spikes occurs 10 days after birth and there is another at the age of about 300
days. It is shown that the first spike is related to viral infections whereas
the second is related to bacterial diseases. By going back to former time
periods during which infant mortality was much higher than currently, it is
possible to get a magnified view of these peaks which in turn may give us
useful information about how an organism adapts to new conditions. Apart from
pathogens, the same methodology can be used to study the response to changes in
other external conditions, e.g. supply of food, temperature or oxygen level.
