Nursing Medical Microbiology is the study of the causes and management of infectious diseases. These can be caused by viruses, bacteria, microfungi and protozoa. Nursing Medical Microbiology may overlap with parasitology, generally considered to be the study of diseases caused by multicellular parasites. Most of the microbes that live on or in humans do no harm. Indeed, they may be positively beneficial. The relationship is, however, finely balanced. Microbes are continually probing our defences and commensals that get into the "wrong" place can do untold damage. Peritonitis, for example is the life-threatening infection that results when gut microbes gain access to the peritoneal cavity, for example following a ruptured appendix. Urinary tract infections are most frequently caused when gut organisms or the skin flora gain access to the bladder. This is why, considering comparative anatomy, women are much more likely to suffer cystitis than men, for example. Some commensals can cause disease because of our life-styles. Dental decay in the Developed world is the result of metabolism of sugars in our diet by our oral flora to produce acid that subsequently etches our teeth.