Both human pathogens and normal microbiota are typically classified as microorganisms.

Human pathogen

AN human pathogen it is a pathogen (microbe or microorganism as a virus, bacterium, prionor fungus) what cause illness at the humans.

The human physiological defense against common pathogens (such as pneumocystis) is primarily the responsibility of the Imune system with the help of some of the body’s normal organs Flora and fauna. However, if the immune system or the “good” microbiota are damaged in any way (such as by chemotherapy, human immunodeficiency virus (HIV) or antibiotics being taken to kill other pathogens), pathogenic bacteria that were being kept under control can proliferate and cause damage to the host. Such cases are called opportunistic infections.

Some pathogens (such as bacteria Yersinia pestiswhat could have caused the black PlagueThe Smallpox virus and the malaria protozoa) were responsible for a large number of victims and had numerous effects on affected groups. Of particular importance in modern times is HIV, which is known to have infected several million humans worldwide, along with the the flu virus. Today, although many medical advances have been made to protect against pathogen infections through the use of vaccination, antibioticsand fungicide, pathogens continue to threaten human life. Social advances like food safety, hygieneand water treatment reduced the threat of some pathogens.



Other information: viral disease

Pathogenic viruses are mainly those from the families of: adenoviridae, Picornaviridae, Herpesviridae, Hepadnaviridae, Coronaviridae, flaviviridae, Retroviridae, Orthomyxoviridae, Paramyxoviridae, Papovaviridae, polyomavirus, Poxviridae, Rhabdoviridaeand Togaviridae. Some notable pathogenic viruses cause smallpox, the flu, mumps, measles, chickenpox, ebolaand rubella. Viruses generally range between 20 and 300 nanometers in length.

This type of pathogen is not cellular and is instead composed of RNA (ribonucleic acid) or DNA (Deoxyribonucleic acid) inside a protein shell – the capsid. Pathogenic viruses infiltrate host cells and manipulate organelles within the cell, such as the ribosomes, Golgi apparatusand endoplasmic reticulum in order to multiply, which usually results in the death of the host cell by cell decay. All the viruses that were contained in the lipid bilayer of the cell membrane are then released into the intercellular matrix to infect neighboring cells to continue the viral life cycle.

The White blood cells are responsible for swallowing the virus using a mechanism known as endocytosis inside extracellular matrix to reduce and fight infection. The components inside the white blood cell are responsible for destroying the virus and recycling its components for the body to use.[citation needed]


Main article: pathogenic bacteria

Photomicrograph of stool with shigella dysentery. This bacteria commonly causes foodborne illness.

Although the vast majority of bacteria are harmless or beneficial to the body, some pathogenic bacteria can cause infectious diseases. The most common bacterial disease is tuberculosiscaused by bacteria Mycobacterium tuberculosis, which affects about 2 million people mainly in sub-Saharan Africa. Pathogenic bacteria contribute to other diseases of global importance, such as pneumoniawhich can be caused by bacteria such as streptococcus and Pseudomonasand foodborne illnesseswhich can be caused by bacteria such as Shigella, campylobacterand salmonella. Pathogenic bacteria also cause infections such as tetanus, typhoid fever, diphtheria, syphilisand hansen’s disease. They typically range between 1 and 5 micrometers in length.[citation needed]


Main article: pathogenic fungi

Fungi are a eukaryotic kingdom of microbes that are generally saprophytes, but can cause disease in humans. Life-threatening fungal infections in humans most often occur in immunocompromised patients or vulnerable people with a weakened immune system, although fungi are common problems in the immunocompetent population as causative agents of skin, nail or fungal infections. Most antibiotics that work on bacterial pathogens cannot be used to treat fungal infections because fungi and their hosts have eukaryotic cells. Most clinical fungicides belong to the azole group. The typical size of fungal spores is 1 to 40 micrometers in length.

Other parasites

Main article: human parasites

Protozoa are unicellular eukaryotes that feed on microorganisms and organic tissues. Considered as “single-celled animals”, as they have animal-like behaviors such as motility, predation, and lack of a cell wall. Many protozoan pathogens are considered human parasites as they cause a variety of diseases, such as: malaria, amebiasis, babesiosis,
giardiasis, toxoplasmosis, cryptosporidiosis, trichomoniasis, Chagas disease, leishmaniasis, african trypanosomiasis (sleeping sickness), acanthamoeba keratitisand primary amoebic meningoencephalitis (naegleriasis).

Two pinworms next to a ruler, measuring 6 millimeters in length

Parasitic worms (Helminths) are macroparasites that can be seen with the naked eye. Worms live and feed on their living host, receiving nutrition and shelter while affecting how the host digests nutrients. They also manipulate the host’s immune system, secreting immunomodulatory products that allow them to live in the host for years. Many parasitic worms are most commonly intestinal worms that are transmitted through the soil and infect the digestive tract; other parasitic worms are found in the blood vessels of the host. The parasitic worms that live in the host can cause weakness and even lead to many illnesses. Parasitic worms can cause many diseases in humans and animals. helminthiasis (worm infection), ascariasisand enterobiasis (pinworm infection) are few caused by various parasitic worms.[citation needed]


Main article: prion

Magnified 100X and stained. This photomicrograph of brain tissue shows the presence of prominent spongiotic changes in the cortex with loss of neurons in a case of variant Creutzfeldt-Jakob disease (vCJD)

prions are misfolded proteins that are transmissible and can influence the abnormal folding of normal proteins in the brain. They do not contain any DNA or RNA and cannot replicate, except to convert already existing normal proteins into the misfolded state. These abnormally folded proteins are characteristically found in many neurodegenerative diseases, as they aggregate in the central nervous system and create plaques that damage tissue structure. This essentially creates “holes” in the fabric. Prions have been found to transmit in three ways: acquired, familial, and sporadic. It has also been discovered that plants play the role of vectors for prions. There are eight different diseases that affect mammals caused by prions, such as scrapie, bovine spongiform encephalopathy (mad cow disease) and Feline spongiform encephalopathy (FSE). There are also ten diseases that affect humans such as, Creutzfeldt-Jakob disease (CJD). and fatal familial insomnia (FFI).

animal pathogens

Animal pathogens are disease-causing agents of wild and domestic animal species, sometimes including humans.


Virulence (the tendency of a pathogen to damage the fitness of a host) evolves when that pathogen can spread from a diseased host, despite that host being very debilitated. An example is the malaria parasite, which can spread from a dying person, hitching a ride to a healthy person on a mosquito that has bitten the sick person. this is called horizontal transmission in contrast to vertical transmissionwhich tends to evolve in symbiosis (after a period of high morbidity and mortality in the population) by linking the evolutionary success of the pathogen to the evolutionary success of the host organism.

evolutionary medicine found that in horizontal transmission, the host population may never develop tolerance to the pathogen.


Transmission of pathogens occurs by many different routes, including airborne, direct or indirect contact, sexual contact, blood, breast milk or other body fluids, and the fecal-oral route. One of the main ways that food or water becomes contaminated is by releasing untreated sewage into a potable water supply or on farmland, resulting in infection of people who eat or drink from contaminated sources. At the developing countries, most of the sewage is discharged into the environment or crops; even in developed countriessome locations experience periodic system failures that result in sewage overflow.


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