Petri net

Infection is graphical mathematically.

N=(P,T,F)

N is a finite ordered list of elements. P and T are disjoint finite sets of places 〇 and transitions □, respectively.

F⊆(P×T)∪(T×P)

This is quite chaotic, so viruses pass through places.
You are closed, so dead or alive is uncertain. We still use masks to decrease possibility blindly.