Immune system

Biological system protecting an organism against disease

The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, such as viruses, bacteria, and parasites, as well as cancer cells and foreign objects, such as wood splinters—distinguishing them from the organism's own healthy tissue. Many species have two major subsystems of the immune system.

From Wikipedia, under CC BY-SA. More on occurri.