  We consider the following interacting particle system: There is a ``gas'' of
particles, each of which performs a continuous time simple random walk on the
d-dimensional lattice. These particles are called A-particles and move
independently of each other. We assume that we start the system with a Poisson
number of particles at each lattice site x, with the number of particles at
different x's i.i.d. In addition, there are a finite number of B-particles
which perform the same continuous time simple random walks as the A-particles.
A- and B-particles are interpreted as individuals who are healthy or infected,
respectively. The B-particles move independently of each other. The only
interaction is that when a B-particle and an A-particle coincide, the latter
instantaneously turns into a B-particle. Let B(t) be the set of sites visited
by a B-particle during [0,t]. We show that B(t) grows linearly in time and has
an asymptotic shape; more precisely, there exists a non-random convex, compact
set B_0 such that almost surely, for all 0 < a <1, (1-a)tB_0 is contained in
B(t) and B(t) is contained in (1+a)tB_0 eventually.
