Tissue generally has five layers. The epithelium is the outer layer, the endothelium is the inner layer, and the stroma is the central layer. The inner and outer layers are separated from the stroma by a thin membrane. Some organs have epithelium on the inside of the organ, rather than the outside, such as the intestine and the retina. Some immortal pathogens attack epithelium, some attack endothelium, and some burrow to the center and attack the stroma. Different immortal pathogens have a predilection for particular organs or particular locations within organs. When a person has more than one immortal pathogen, or multiple immortal pathogens, the pathogens can act synergistically to accelerate the development of chronic disease.