Urban environments are characterized by the density of buildings and elements of a number of infrastructures that support urban residents in their daily life. These built elements and the activities that take place within towns and cities create a distinctive climate and increase air, water, and soil pollution. Within this context the elements of the natural environment that either are residual areas representative of the pre-urbanized area or are created by people contain distinctive floral and faunal communities that do not exist in the wild. The diverse prions, viruses, micro-organisms, plants, and animals that live there for all or part of their life cycle and their relationships with each other and with humans are illustrated with examples of diseases, parasites, and pests. Plants and animals are found inside as well as outside buildings. The roles of plants inside buildings and of domestic and companion animals are evaluated. Temporal and spatial distribution patterns of plants and animals living outside buildings are set out and generalizations are drawn, while exceptions are also discussed. The strategies used and adaptions (genotypic, phenotypic, and behavioural) adopted by plants and animals in face of the challenges presented by urban environments are explained. The final two chapters contain discussions of the impacts of urban environments on human biology and how humans might change these environments in order to address the illnesses that are characteristic of urbanites in the early twenty-first century.