  We study infection spread among biased random walks on $\mathbb{Z}^{d}$. The
random walks move independently and an infected particle is placed at the
origin at time zero. Infection spreads instantaneously when particles share the
same site and there is no recovery. If the initial density of particles is
small enough, the infected cloud travels in the direction of the bias of the
random walks, implying that the infection does not survive locally. When the
density is large, the infection spreads to the whole $\mathbb{Z}^{d}$. The
proofs rely on two different techniques. For the small density case, we use a
description of the infected cloud through genealogical paths, while the large
density case relies on a renormalization scheme.
