 A human's microbiota, the other microorganisms living within and on the body, compete with pathogenic organisms and play a large role in pathogenic control.Lastly, a semi-permeable membrane known as the blood-brain-barrier is a lining of endothelial cells separating the blood from the tissues and organs.Without this lining, viruses and bacteria could easily infect vital human organs such as the brain, lungs, and placenta.