Groups of specialized cells cooperate to form a tissue, such as a muscle. Different tissues are in turn grouped together to form larger functional units, called organs. Each type of cell, tissue, and organ has a distinct structure and set of functions that serve the organism as a whole. The human organism has systems for digestion, respiration, reproduction, circulation, excretion, movement, control and coordination, and for protection from disease. Disease represents a breakdown in structures or functions of an organism.
Some diseases are the result of intrinsic failures of the system.
Others are the result of infection by other organisms. Some organisms reproduce asexually. Other organisms reproduce sexually. In many species, including humans, females produce eggs and males produce sperm. An egg and sperm unite beginning the development of a new individual.