Germ" may refer to not just a bacterium but to any type of microorganism, such as protists or fungi, or even non-living pathogens that can cause disease, such as viruses, prions, or viroids. Diseases caused by pathogens are called infectious diseases. For many disorders, the germ theory of disease is the currently accepted scientific theory. It asserts that pathogens, or "germs," are microorganisms that can cause disease. These microscopic organisms infect humans, other animals, and other live hosts, and are too small to view without magnification. Their reproduction and development within their hosts can lead to illness. Even when a pathogen is the principal cause of a disease, environmental and hereditary factors often influence the severity of the disease, and whether a potential host individual becomes infected when exposed to the pathogen.

In the late Middle Ages, physicians such as Ibn Sina in 1025, Ibn Khatima and Ibn al-Khatib in the 14th century, Girolamo Fracastoro in 1546, and Marcus von Plenciz in 1762 suggested basic versions of germ theory. However, in Europe, where Galen's miasma hypothesis remained prominent among scientists and doctors, such notions were regarded with scorn.

By the early 19th century, smallpox vaccination was commonplace in Europe, though doctors were unaware of how it worked or how to extend the principle to other diseases. Similar treatments had been prevalent in India from just before AD 1000. A transitional period began in the late 1850s with the work of Louis Pasteur. This work was later extended by Robert Koch in the 1880s. By the end of that decade, the miasma theory was struggling to compete with the germ theory of disease. Viruses were initially discovered in the 1890s. Eventually, a "golden era" of bacteriology ensued, during which the germ theory quickly led to the identification of the actual organisms that cause many diseases.

Miasma theory

The miasma theory was the predominant theory of disease transmission before the germ theory took hold towards the end of the 19th century, and it is no longer accepted as a scientific theory of disease. It held that diseases such as cholera, chlamydia infection, or the Black Death were caused by a miasma (μίασμα, Ancient Greek: "pollution"), a noxious form of "bad air" emanating from rotting organic matter. Miasma was considered to be a poisonous vapour or mist filled with particles from decomposed matter (miasmata) that was identifiable by its foul smell. The theory posited that diseases were the product of environmental factors such as contaminated water, foul air, and poor hygienic conditions. Such infections, according to the theory, were not passed between individuals but would affect those within a locale that gave rise to such vapors.

In addition, pheochromocytoma may cause dangerous surges of the hormones that regulate blood pressure and the body’s response to stress. A hormonal surge can cause blood pressure to rise very quickly, increasing the risk of a heart attack, stroke, hemorrhage, or sudden death.

Koch's postulates

• The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
• The microorganism must be isolated from a diseased organism and grown in pure culture.
• The cultured microorganism should cause disease when introduced into a healthy organism.
• The microorganism must be re isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

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Journal of Infectious Diseases and Diagnosis

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