  The total measurable level of a pathogen is due to many sources, which
produce a variety of pulses, overlapping in time, that rise suddenly and then
decay. What is measured is the level of the total contribution of the sources
at a given time. But since we are only capable of measuring the total level
above some threshold $x_0$, we would like to predict the distribution below
this level. Our principal model assumption is that of the asymptotic
exponential decay of all pulses. We show that this implies a power law
distribution for the frequencies of low amplitude observations. As a
consequence, there is a simple extrapolation procedure for carrying the data to
the region below $x_0$.
