<unk>  refers to bacteria (not viruses) known or suspected to cause<unk> .  Some of these may in fact be opportunists:  bacteria present in the environment that colonize a host made vulnerable by<unk> .  As such,<unk>  would be the cause, and bacterial infections the effect, rather than the other way around.
In 1890, German bacteriologist Robert Koch defined four requirements to prove that a given organism causes a given disease.  These requirements, known as Koch's postulates, guide research into the bacterial cause of many diseases, including<unk> , and establish a gold standard of evidence consistent with "proof".
Cancer bacteria
Gram-negative bacteria
Aerobic organism
RNA virus
Microorganism
Parasitic worm
Anaerobic organism
Metal