We propose a minimal model of locally-activated diffusion, in which the diffusion coefficient of a one-dimensional Brownian particle is modified in a prescribed way -either increased or decreased -upon each crossing of the origin. Such a local mobility decrease arises in the formation of atherosclerotic plaques due to diffusing macrophage cells accumulating lipid particles. We show that spatially localized mobility perturbations have remarkable consequences on diffusion at all scales, such as the emergence of a non-Gaussian multi-peaked probability distribution and a dynamical transition to an absorbing static state. In the context of atherosclerosis, this dynamical transition can be viewed as a minimal mechanism that causes macrophages to aggregate in lipid-enriched regions and thereby to the formation of atherosclerotic plaques. Many-particle systems that consume energy for selfpropulsion -active particle systems -have received growing attention in the last decade, both because of the new physical phenomena that they display and their wide range of applications. Examples include molecular motors, cell assemblies, and even larger organisms [1]. The intrinsic out-of-equilibrium nature of these systems leads to remarkable effects such as non-Boltzmann distributions [2], long-range order even in low spatial dimensions [3] and spontaneous flows [4]. At the single-particle level, the active forcing of a Brownian particle leads to non-trivial statistics. For example, it has been recently shown [5,6] that a random walk which is reset to its starting point at a fixed rate has a non-equilibrium stationary state, as opposed to standard Brownian motion. Another example is given by selfpropelled Brownian particles [7], which can yield sharply peaked probability densities for the particle velocity.In this letter, we consider a new class of problems in which the active forcing of a Brownian particle is localized in space. While the impact of localized perturbations on random walks has been investigated [8], in part because of its relevance to a wide range of situations, such as localized sources and sinks [9,10], trapping [11,12] or diffusion with forbidden [13], hop-over [14] or defective [15] sites, the role of local activation on Brownian-particle dynamics remains open. We present a minimal model of locally activated diffusion, in which the diffusivity of a Brownian particle is modified -either increased or decreased -in a prescribed way upon each crossing of the origin.A prototypical example is a bacterium in the presence of a localized patch of nutrients, which enhances the ability of the bacterium to move, or, alternatively, toxins that impair bacterial mobility. This type of localized decrease of mobility also underlies the dynamics of a cell (1) rapid diffusion of a "free" macrophage cell; (2) upon entering a localized lipid-enriched region, the macrophage accumulates lipids, and thereby grows and becomes less mobile; (3) after many crossings of the lipidenriched region, the macrophage eventually gets trapped, result...