BrazilPopulation Density: Number of individuals in a population relative to space.Rivers: Large natural streams of FRESH WATER formed by converging tributaries and which empty into a body of water (lake or ocean).Seasons: Divisions of the year according to some regularly recurrent phenomena usually astronomical or climatic. (From McGraw-Hill Dictionary of Scientific and Technical Terms, 6th ed)Psychodidae: Small, hairy, moth-like flies which are of considerable public health importance as vectors of certain pathogenic organisms. Important disease-related genera are PHLEBOTOMUS, Lutzomyia, and Sergentomyia.Prevalence: The total number of cases of a given disease in a specified population at a designated time. It is differentiated from INCIDENCE, which refers to the number of new cases in the population at a given time.Insect Vectors: Insects that transmit infective organisms from one host to another or from an inanimate reservoir to an animate host.Trees: Woody, usually tall, perennial higher plants (Angiosperms, Gymnosperms, and some Pterophyta) having usually a main stem and numerous branches.Indians, South American: Individual members of South American ethnic groups with historic ancestral origins in Asia.Socioeconomic Factors: Social and economic factors that characterize the individual or group within the social structure.Endemic Diseases: The constant presence of diseases or infectious agents within a given geographic area or population group. It may also refer to the usual prevalence of a given disease with such area or group. It includes holoendemic and hyperendemic diseases. A holoendemic disease is one for which a high prevalent level of infection begins early in life and affects most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do children (malaria in many communities is a holoendemic disease). A hyperendemic disease is one that is constantly present at a high incidence and/or prevalence rate and affects all groups equally. (Last, A Dictionary of Epidemiology, 3d ed, p53, 78, 80)Bertholletia: A plant genus of the family Lecythidaceae which is the source of edible Brazil nuts.Cross-Sectional Studies: Studies in which the presence or absence of disease or other health-related variables are determined in each member of the study population or in a representative sample at one particular time. This contrasts with LONGITUDINAL STUDIES which are followed over a period of time.Urban Population: The inhabitants of a city or town, including metropolitan areas and suburban areas.Epidemiologic Methods: Research techniques that focus on study designs and data gathering methods in human and animal populations.Simuliidae: Several species of the genus Simulium (family Simuliidae) that act as intermediate hosts (vectors) for the parasitic disease ONCHOCERCIASIS.