In real life, however, even in our worst circumstances we have always been a relatively minor interest of the vast microbial world. Pathogenicity is not the rule. Indeed, it occurs so infrequently and involves such a relatively small number of species, considering the huge population of bacteria on the earth, that it has a freakish aspect. Disease usually results from inconclusive negotiations for symbiosys, an overstepping of the line by one side or the other, a biologic misinterpretation of borders.
Lewis Thomas
The Lives of a Cell, Germs, p76