  Fix a finite undirected graph $\Gamma$ and a vertex $v$ of $\Gamma$. Let $E$
be the set of edges of $\Gamma$. We call a subset $F$ of $E$ pandemic if each
edge of $\Gamma$ has at least one endpoint that can be connected to $v$ by an
$F$-path (i.e., a path using edges from $F$ only). In 1984, Elser showed that
the sum of $\left(-1\right)^{\left| F\right|}$ over all pandemic subsets $F$ of
$E$ is $0$ if $E\neq \varnothing$. We give a simple proof of this result via a
sign-reversing involution, and discuss variants, generalizations and
refinements, revealing connections to abstract convexity (the notion of an
antimatroid) and discrete Morse theory.
