Ecology is the study of the distribution and abundance of organisms and their interactions with their environment, including parasites and pathogens. Immunology is the study of physiological functioning of the immune system in states of health and disease. The two disciplines has unified and led to the new discipline of ecological immunology, or ecoimmunology in integrative perspectives. Ecoimmunology is a rapidly expanding research field. Its prime concern is to understand the ecological questions such as life-history trade-offs, sexual selection, population dynamics and so on in view of the cost of immunity. The immune system, which can protect animals from infection and attack of pathogens in the environment, plays an important role in determining organisms' survival and their fitness. The focus of ecoimmunology has been to examine the causes and consequences of variation in immune function in the context of evolution and of ecology, specifically why and how biotic and abiotic factors contribute to variation in immunity in free-living organisms. Whether immune defense is costly in terms of energy or resources is a basic scientific question in ecoimmunology. Many researchers have demonstrated that immune defense is costly. Resources is not limitless, hence organisms must allocate limited resources among competing, costly physiological functions. Trade-offs occur between immunity and reproduction or growth, in which investing in one particular process, such as reproduction, limits the resources available to other processes, such as somatic growth or fighting a parasitic infection. Parasites were of fundamental importance in the evolution of sexually selected characters and in the maintenance of female preferences for ornamented males. The immunocompetence handicap hypothesis states that testosterone is responsible for the production of male secondary sexual traits and is simultaneously immunosuppressive, the cost of being able to express sexual traits is decreased immune function. Immunosuppression should result in greater vulnerability to pathogen or parasite attack; therefore, only high-quality males could afford to display sexual characteristics fully without suffering large parasite loads. In this paper we review the development, the focuses and the prospects of ecoimmunology.