An important area of research focus for NIAID is the immune system, the complex network of cells, tissues, and
organs that work together to defend the body against attacks by foreign
invaders such
as bacteria, viruses, parasites, and fungi.
The metabolic activities performed by these bacteria resemble those of an
organ, and these microorganisms perform a host of useful functions, such
as training the immune system, attacking foreign
invaders (like food poisoning compounds, toxins, etc.), preventing growth of harmful, pathogenic bacteria in your gut, regulating the development of the intestinal lining, producing vitamins such
as biotin and vitamin K, and even producing hormones (10).