It could be anything that is inside or outside your body. You have immune cells all over the place. I do not subscribe to germ theory, but prefer the terroir model put forth by Bechamps. He opposed Pasteur, who, on his deathbed said, "It IS the terroir", and then he died. But, because of the horrible Rockefellers, medicine has been absolutely taken over by the molecular biologists, and everything is germs, viruses, DNA etc. So supporting the body's natural ability to clear debris and protect itself goes by the wayside, and all we think of is "give me a shot or a pill" to fix me!! This is very bad. I haven't been sick for some time, because I monitor my body and notice any variation on my good health and IMMEDIATELY attend to it and take something that will strengthen my defenses.