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What Are Pathogens?

Pathogens are microorganisms that cause disease in their host. There are five major groups of these pathogens: viruses, bacteria, fungi and protozoa.

Pathogens cause disease through various means, including producing toxins that damage tissues or kill cells within the host’s body. Pathogenicity and virulence are emergent properties which depend on complex interactions among host, environment and microbe.


Viruses are infectious microscopic organisms that can infiltrate plants, animals, bacteria or fungi. Unlike their multicellular cousins (e.g. bacteria or fungi), viruses lack nuclei and do not generate energy through producing Adenosine Triphosphate (ATP). Instead they gain energy by invading cells. Viruses contain genetic material encased in a protective casing known as a capsid, with instructions for entering and reproducing in host cells. Unlike bacteria, viruses rely on host cells for reproduction; their symptoms often serve as means of identification. Many viruses have been named for the place or organ they were first isolated (e.g. Ebola virus from Congo), the disease they cause (herpesvirus), or their discoverer (e.g. Epstein-Barr virus).

Viral infections invade host cells to replicate, then leave by burst of the cell’s membrane and dispersed viral particles into the environment – these in turn infect other cells and reproduce more. They may also spread via touch or respiratory droplets from coughing or sneezing individuals with COVID-19; additionally they can travel via ticks and mosquitoes with Lyme disease and malaria infection attached to their feet or heads, infecting new hosts in this manner.

Obliged pathogens, or those which need a host for reproduction, are known as obligatory pathogens and solely exist to reproduce and spread. Their lives tend to be very complex – for instance some trematodes (flatworms that infest humans and other mammals’ intestines), have three hosts for different stages in their lives (deer ticks, intermediate hosts such as molluscs, and insects such as mosquitoes), while influenza virus has two host cycle. A few pathogens such as herpesviruses even four host life cycles!


Bacteria are single-celled microorganisms that come in various forms such as rods, spirals or spheres. While bacteria are generally larger than viruses with DNA contained within nuclei enclosed within membranes or cell walls that protect their nuclei from outside interference, they still cause disease through secreting toxins which damage tissues and interfere with essential body processes – antibiotics are used to treat such infections.

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Some bacteria are naturally found within our bodies and do not make people sick, while others enter from outside and proliferate undetected or eliminated. Pathogens are the bacterial microbes with the capacity to cause the most harm and disease; they may produce toxins to attack from within, or induce an immune response which damages healthy tissue as well as attacking pathogens.

Most pathogens cause disease through the production of toxins. These toxins allow bacteria to reach new tissues or cells, or block out an individual from expelling them; some bacterial toxins are among the most lethal known poisons, including those responsible for tetanus or anthrax infections.

Pathogenic bacteria depend on various variables, including genetic makeup and environment in which they reside. Their ability to cause disease depends on virulence – the characteristics that allow the microbes to debilitate host cells while attacking specific targets – and this ultimately determines their pathogenicity.

Humans typically encounter bacteria such as staphylococci or Candida as the most prevalent pathogens; some strains can even become pathogenic if their numbers increase beyond control and overwhelm an immune system; this form of infection is commonly referred to as an “opportunistic infection.”

Others pathogens include fungi that thrive on skin or mouth surfaces and cause infections such as athlete’s foot or ringworm; protozoa, large single-celled organisms capable of causing diseases like malaria or sleeping sickness; parasites (louse and tapeworms); and pathogens which enter our bodies via airborne droplets or food or water contamination – though more dangerous infections like rabies or swine flu may spread between animals and people as well.

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Fungi are single-celled organisms found both free-living in soil or water environments and parasitic or symbiotically with other organisms, including yeasts, rusts, mildews, molds and mushrooms. Fungi constitute a vast kingdom of organisms that includes yeasts, rusts, mildews molds and mushrooms – with some serving as decomposers while others cause diseases in humans or animals such as athletes foot and ringworm. Mycorrhizal fungi form relationships with plants for mutually beneficial relationships symbiosis with plants in mycorrhizal relationships.

Fungal pathogens can also spread through direct physical contact between an infected individual, animal, or object and others, which includes coughs or sneezes, eating contaminated food, touching an infected surface or petting a sick animal. Fungi may also spread via airborne spores or water in humid environments; parasitic strains of this fungi have even been observed living within animals like tigers or primates found at zoos or even within bloodstreams of dogs cats horses or people!

Pathogens typically produce toxins to damage tissues and suppress immunity, with some species even producing deadly strains that can quickly kill their victims. The severity of disease symptoms produced is known as “virulence”.

Fungi are responsible for numerous diseases, which can often be prevented or treated using medication. Some are found naturally on our skin, like Candida albicans (yeast). Others can spread through public spaces or eating raw vegetables, meats or dairy products that contain them. Fungal infections can be avoided by washing hands regularly after using the bathroom, changing a child’s diaper, petting an animal etc.

Fungi are key elements in our environment, helping plants access water and nutrients from soil, as well as creating new materials used by building industries. Fungi can also help break down pollutants, produce biofuels and be used as medical treatments – they even predict conditions that threaten survival allowing scientists to create more effective strategies against them. Researchers are investigating whether some fungi possess an evocative form of memory which might enable them to respond more quickly in response to environmental threats.

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Parasites, as their name implies, are organisms that live off or inside another living being (called their host) for mutual gain – in exchange for food, shelter and/or protection from them in return. A parasite’s infection can range from mild discomfort to death and there are various species from single-celled organisms to visible worms that cause illness in people, animals and plants alike – from common colds to Ebola virus outbreaks.

Parasites can either be obligate pathogens, which require infecting a host to complete their lifecycle, or facultative pathogens that do not need one for survival. Obligate pathogens possess highly specialized genes called “virulence factors” to manipulate immune systems, metabolisms and behavior within host bodies – these virulence factors also known as parasites’ signature characteristics.

Fungi are among the most prevalent obligate pathogens found in humans, causing illnesses like athlete’s foot and ringworm. Some strains also produce toxins to increase their virulence and capacity to cause illness.

Protist pathogens infect plants and cause them to rot, while others infiltrate animals like mosquitoes that transmit malaria. Furthermore, some protists serve as obligate pathogens that live in both people’s intestines as well as animals’ like Entamoeba histolytica which causes diarrhea; others act as parasites like lice or tapeworms.

Parasites can spread through contaminated water, food or air. Giardia duodenalis, Cryptosporidium parvum and Cyclospora cayetanensis are three common causes of diarrhea; Entamoeba histolytica causes digestive distress; while Toxoplasma gondii can spread via organ transplantation or blood transfusions. Most parasitic infections can be avoided through practicing good hygiene, including frequent handwashing – particularly before eating, after using the restroom and when handling pet feces – and vaccination against some parasites. Some medications or other treatments, like cutting away affected skin may also help protect you; it is best to speak with a healthcare provider regarding what treatment will work best in your situation.