Pathogen – Online Notes – NCERT Solutions for Biology Class 12
In biology, a pathogen (Greek pathos means “suffering, pain ” and genēs means “producer of”) in the oldest and broadest sense, a pathogen is anything that can produce disease; the term came into use in the 1880s. Typically the term is used to describe an infectious agent such as a virus, bacterium, protozoa, prion, a fungus, or another microorganism.
We define a pathogen as a biological agent that causes disease or illness to its host (an organism which is attacked by the pathogen).
Most often the pathogens disrupt the normal physiology of a multi-cellular animal or plant. However, pathogens can infect unicellular organisms from all of the biological kingdoms. There are several substrates and pathways whereby pathogens can invade a host. The human body contains many natural defenses against some of the common pathogens in the form of the human immune system and by some “helpful” bacteria present in the human body’s normal flora. Some pathogens have been found to be responsible for massive amounts of casualties and have had numerous effects on afflicted groups. Today, while many medical advances have been made to safeguard against infection by pathogens, through the use of vaccination, antibiotics, and fungicide, pathogens continue to threaten human life. Social advances such as food safety, hygiene, and water treatment have reduced the threat from some pathogens.
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Pathogens have evolved specific mechanisms for interacting with their hosts
The human body is a complex and thriving ecosystem. It contains about 1013 human cells and also about 1014 bacterial, fungal, and protozoan cells, which represent thousands of microbial species. These microbes, called the normal flora, are usually are found in certain areas of the body, including the skin, mouth, large intestine, and vagina. In addition, humans are always infected with viruses, most of which rarely, if ever, become symptomatic. If it is normal for us to live in such close intimacy with a wide variety of microbes, how is it that some of them are capable of causing us illness or death?
Pathogens are usually distinct from the normal flora. Our normal microbial inhabitants only cause trouble if our immune systems are weakened. The pathogens have developed highly specialized mechanisms for crossing cellular and biochemical barriers and for eliciting specific responses from the host organism that contribute to the survival and multiplication of the pathogen.
In order to survive and multiply in a host, a successful pathogen must be able to:
(1) Colonize the host
(2) Find a nutritionally compatible niche in the host body.
(3) Avoid, subvert, or circumvent the host innate and adaptive immune responses.
(4) Replicate, using host resources and
(5) Exit and spread to a new host.
Pathogens have evolved mechanisms that maximally exploit the biological machinery of their host organisms. They totally depend on their host for their survival.
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Pathogens and Diseases
Although we can easily understand why infectious microorganisms would evolve to reproduce in a host, it is less clear why they would evolve to cause disease. One explanation is that, in some cases, the pathological responses elicited by microorganisms enhance the efficiency of their spread or propagation and hence clearly have a selective advantage for the pathogen. The virus-containing lesions on the genitalia caused by herpes simplex infection, for example, facilitate the direct spread of the virus from an infected host to an uninfected partner during sexual contact. Similarly, diarrheal infections are efficiently spread from patient to caretaker. In many cases, however, the induction of disease has no apparent advantage for the pathogen.
Many of the symptoms and signs that we associate with infectious disease are direct manifestations of the host’s immune responses in action. Some hallmarks/ symptoms of bacterial infection, are swelling and redness at the site of infection and the production of pus (mainly dead white blood cells), are the direct result of immune system cells attempting to destroy the invading microorganisms. Fever, too, is a defensive response, as the increase in body temperature can inhibit the growth of some microorganisms. Thus, understanding the biology of an infectious disease requires an understanding of both pathogen and host.
For notes on Types of Pathogens, Biology notes for Class 12.
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