A plot of the failure rate h(t) over time (t) for most products yields a curve that looks like a drawing of a bathtub with 3 distinct regions.
Early Failure Period (Infant Mortality Period)
Intrinsic Failure Period (Stable Failure Period)
Wearout Failure Period
The Bathtub Curve also applies to lifetime health of a human being. However, the Intrinsic Failure Period of human health is not quite stable with a constant failure rate level. Instead, the period comprises swings, aside from occasional illnesses.
The situation is analogous to wobbles in a spinning top. There are relatively short periods of instability during the Intrinsic Failure Period, but the top makes a comeback to stability despite a reduced spinning rate due to friction with the floor.
I have observed that my house plants almost die every few years shedding many leaves at once. After the near-death event, the plants seem to experience a sort of "renewal", going stronger as ever.
In my case, I bought a pair of reading glasses at age 42. After a brief period, however, I recovered my LASIK-enhanced 20/20 vision, and have no need for reading glasses. The muscles to focus my reading vision had experienced a short period of instability.
