Physica A 342 (2004) 249–255 www.elsevier.com/locate/physa Critical behavior of a vector-mediated propagation of an epidemic process E. Macnadbay a ; ∗ , R. Bezerra a , U.L. Fulco b , M.L. Lyra c , C. Argolo a a Departamento de F sica, Centro Federal de Educac ˜ ao Tecnol ogica de Alagoas, 57020-510 Macei o-AL, Brazil b Departamento de F sica, Universidade Federal do Piaui, 64049-550, Teresina-PI, Brazil c Departamento de F sica, Universidade Federal de Alagoas, 57072-970 Macei o-AL, Brazil Received 24 October 2003; received in revised form 19 January 2004 Available online 18 May 2004 Abstract We investigate the critical behavior of a model that mimics the propagation of an epidemic process over a population mediated by a density of diusive individuals which can infect a static population upon contact. We simulate the above model on nite chains to determine the critical density of vectors above which the system achieves a stationary active state with a nite density of infected individuals. Further, we employ a scaling analysis to determine the order parameter, correlation length and critical relaxation exponents. We found evidences that this model does not belong to the usual direct percolation universality class. c  2004 Published by Elsevier B.V. PACS: 64.60.Fr; 64.60.Ak; 87.23.Cc Keywords: Absorbing state phase-transition; Directed percolation; Diusion-limited reaction The critical behavior of non-equilibrium systems describes relevant features of several phenomena in physics, chemistry and biology [1]. In general these systems present a second-order phase transition between a vacuum state, where the order parameter density vanishes, and a steady reactive state [2]. At high dimensions, where uctuations can be neglected, these systems can be modelled by a set of mean-eld-like dierential * Corresponding author. 0378-4371/$ - see front matter c  2004 Published by Elsevier B.V. doi:10.1016/j.physa.2004.04.085