Germ theory of disease
The germ theory of disease is the currently accepted scientific theory for many diseases. It states that microorganisms known as pathogens or "germs" can lead to disease. These small organisms, too small to see without magnification, invade humans, other animals, and other living hosts. Their growth and reproduction within their hosts can cause disease. "Germ" may refer to not just a bacterium but to any type of microorganism or even non-living pathogens that can cause disease, such as protists, fungi, viruses, prions, or viroids.[ Diseases caused by pathogens are called infectious diseases. 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.
Viruses are some of the most mysterious organisms on Earth. They’re among the world’s tiniest lifeforms, and because none can survive and reproduce without a host, some scientists have questioned whether they should even be considered living things. Now, scientists have discovered one that has no recognizable genes, making it among the strangest of all known viruses. But how many viruses do we really know? Another group has just discovered thousands of new viruses hiding out in the tissues of dozens of animals.
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