In this new century mankind faces ever more challenging environmental and publichealthproblems, suchaspollution, invasionbyexoticspecies, theem- gence of new diseases or the emergence of diseases into new regions (West Nile virus, SARS, Anthrax, etc.), andtheresurgenceofexistingdiseases(in?uenza, malaria, TB, HIV/AIDS, etc.). Mathematical models have been successfully used to study many biological, epidemiological and medical problems, and nonlinear and complex dynamics have been observed in all of those contexts. Mathematical studies have helped us not only to better understand these...