  One of the most notable features in repulsive particle based active matter
systems is motility-induced-phase separation (MIPS) where a dense, often
crystalline phase coexists with a low density fluid. In most active matter
studies, the activity is kept constant as a function of time; however, there
are many examples of active systems in which individual particles transition
from living or moving to dead or nonmotile due to lack of fuel, infection, or
poisoning. Here we consider an active matter particle system at densities where
MIPS does not occur. When we add a small number of infected particles that can
effectively poison other particles, rendering them nonmotile, we find a rich
variety of time dependent pattern formation, including MIPS, a wetting phase,
and a fragmented state formed when mobile particles plow through an nonmotile
packing. We map out the patterns as a function of time scaled by the duration
of the epidemic, and show that the pattern formation is robust for a wide range
of poisoning rates and activity levels. We also show that pattern formation
does not occur in a random death model, but requires the promotion of
nucleation by contact poisoning. Our results should be relevant to biological
and active matter systems where there is some form of poisoning, death, or
transition to nonmotility.

Soft Condensed Matter;Statistical Mechanics