MICROBIAL ECOLOGY. There exist “dozens of polymicrobial diseases, caused by combinations of viruses, bacteria, fungi, and parasites. In these infections, the presence of one micro-organism generates a niche for other pathogenic micro-organisms to colonise, one micro-organism predisposes the host to colonisation by other micro-organisms, or two or more non-pathogenic micro-organisms together cause disease.”
http://mpkb.org/home/pathogenesis/kochs_postulates