  We study atypical behavior in bootstrap percolation on the Erd\H{o}s-R\'enyi
random graph. Initially a set $S$ is infected. Other vertices are infected once
at least $r$ of their neighbors become infected. Janson et al. (2012) locates
the critical size of $S$, above which it is likely that the infection will
spread almost everywhere. Below this threshold, a central limit theorem is
proved for the size of the eventually infected set. In this note, we calculate
the rate function for the event that a small set $S$ eventually infects an
unexpected number of vertices, and identify the least-cost trajectory realizing
such a large deviation.
