N:	Total number of subsystems in one organism
F(t):	The number of failed subsystems at time t, $F(t)=\frac{N·F_0}{F_0+(N-F_0)e^{-rNt}}$
M(t):	The number of subsystems which are functioning correctly at time t, $M(t)=N-F(t)$
$\tau$:	inflection point $\tau =1/(rN)·ln(N/F_0-1)$, such that $F(\tau )=N/2$
$\mu (t)$:	mortality rate, $\mu (t)\propto \frac{F_0}{F_0+(N-F_0)e^{-rNt}}$ In the limit of small fractions of failed subsystems (small F(t)/N): $\mu (t)\approx R_0{e}^{\mathit{bt}}$ where $b=rN$