ABSTRACT
Microbiology is the study of bacteria, fungi, parasites, and viruses; these are microorganisms that are too small to be seen with the naked eye. Microorganisms that are harmful to human health are referred to as pathogens, that is, organisms that are capable of causing disease. Most bacteria, however, are not pathogens, and it has been estimated that nonpathogenic or harmless bacteria make up over 70% of all bacteria. It is also estimated that about 100 trillion mostly benign bacteria-called normal ora-are found symbiotically in the human body, primarily in the oral mucosa, intestines, and surface of the skin. Bacterial cells consequently outnumber human cells by a factor of 10 to 1. Normal ora in the body has created the recently discovered human microbiome, an ecosystem that has been linked not only to the digestion of food but also to obesity, depression, asthma, Crohn’s disease, and even cancer.1
