Let me try to fill in. Prior infection with something can leave behind genetic changes (which can also be passed to offspring, even across many generations). Th

Let me try to fill in. Prior infection with something can leave behind genetic changes (which can also be passed to offspring, even across many generations). The changes may or may not cause a disorder. Subsequent infection by a similar agent may cause the immune system to "learn" aspects of that agent. This can then cause the immune system to identify similar aspects of host cells, and mount a response to the perceived threat. Relatedly, allergies are also a class of immune disorder based on misidentification. For instance, toxicodendron plants like "poison ivy" produce a harmless substance that is capable of chemically bonding to human cell membranes, which does not seem to impair the primary function of the cell, but does alter its ability to respond correctly when "interrogated" by relevant T-cells. These T-cells are immune cells which can be described as drifting through the body, randomly attaching to other cells, and then asking those cells to authenticate. The response from an affected cell is malformed, causing the T-cell to release a chemical signal which is like say "Kill everything here!". This causes other immune cells which enter the area to commence destroying friend and foe alike. More immune cells will continue to be recruited for as long as the 'kill' signal lasts. The signal will continue to be produced each time another cell fails to authenticate.