What kind of pathogens are there

The vast majority of microbes are harmless to us, and many play essential roles in plant, animal and human health. Others, however, are either obligate or facultative pathogens exerting a spectrum of deleterious effects on their hosts. Infectious diseases have historically represented the most common cause of death in humans until recently, exceeding by far the toll taken by wars or famines. From the dawn of humanity and throughout history, infectious diseases have shaped human evolution, demography, migrations and history.

A pathogen is defined as an organism causing disease to its host, with the severity of the disease symptoms referred to as virulence. Pathogens are taxonomically widely diverse and comprise viruses and bacteria as well as unicellular and multicellular eukaryotes. Every living organism is affected by pathogens, including bacteria, which are targeted by specialized viruses called phages. The number of viruses and bacteria on earth is staggering and they occupy essentially every environment.

A liter of surface seawater typically contains in excess of ten billion bacteria and billion viruses. The number of viruses on Earth is estimated to be around 10 31 , which corresponds to roughly ten billion times the number of stars in the universe [ 1 ]. An average human is made up of about 30 trillion cells but carries a similar number of bacteria, mostly in the gut [ 2 ]. The vast majority of viruses and bacteria we are exposed to have no negative effect and some can even be beneficial, though a tiny fraction of these can severely affect our health.

Specifically, about one in a billion microbial species is a human pathogen. Indeed, approximately human pathogens have been described, whereas it has been estimated that there are one trillion microbial species on Earth, the vast majority of which remain uncharacterized [ 1 ]. Pathogens can be divided into two main categories, namely facultative and obligate pathogens, reflecting how intimately their life cycle is tied to their host.

Facultative pathogens are organisms for which the host is only one of the niches they can exploit to reproduce. Facultative pathogens are primarily environmental bacteria and fungi that can occasionally cause infection. They include many of the most problematic hospital-acquired bacteria involved in the antimicrobial resistance pandemic. A distinction is sometimes made between facultative and accidental pathogens, with the latter representing those which only occasionally infect weakened or immunocompromised hosts.

Obligate pathogens require a host to fulfil their life cycle. All viruses are obligate pathogens as they are dependent on the cellular machinery of their host for their reproduction. Obligate pathogens are found among bacteria, including the agents of tuberculosis and syphilis, as well as protozoans such as those causing malaria and macroparasites. Some obligate pathogens require multiple different hosts to fulfil their life cycle.

The definite host, which supports the adult form of the pathogen, is often a vertebrate and the intermediate host referred to as a vector is generally an arthropod or a mollusc. This alternation of vertebrate and invertebrate hosts is found in viruses for example the Zika virus , bacteria for example Lyme disease and protozoa malaria.

Trematodes parasitic flatworms go even further and some exhibit among the most baroque life cycles. Digenetic trematodes have a basic three-host life cycle, and for some species a four-host life cycle. For instance, Halipegus occidualis sequentially has to infect a freshwater snail, an ostracod, a dragonfly nymph and ends its cycle after the dragonfly is eaten by the green frog Rana clamitans , where it resides under its tongue [ 3 ]. Some pathogens are limited to infecting a single host species, whereas others can infect a multitude of host species.

Host ranges can feel highly idiosyncratic if not outright puzzling. For example, leprosy in humans is caused by two related intracellular bacteria Mycobacterium leprae and Mycobacterium lepromatosis , which are essentially restricted in the wild to humans, as well as armadillos in the Americas and red squirrels in Scotland [ 4 ].

Conversely, Yersinia pestis , another intracellular obligate bacterium and the agent of plague, has a natural life cycle involving alternating infections of rodents and fleas, but can infect essentially any mammalian host.

An interesting twist in the case of plague is that Y. With the exception of uncommon occurrences of human-to-human transmissions, referred to as pneumonic plague, plague epidemics bubonic plague are caused by plague-infected fleas biting humans. Somewhat ironically for a pathogen that is possibly the biggest killer in human history, bubonic plague is a complete evolutionary disaster.

The human host is at a very high risk of dying, the flea cannot reproduce on a meal of human blood and the bacterium is stuck in an evolutionary dead-end as it cannot transmit to another host. There is no obvious predictor for the host range of different pathogens. Intuitively, it may be tempting to predict that pathogens with a more intimate relationship with their host are more closely adapted to their host, and thus have a more restricted host range.

Also, intracellular bacteria do not seem to have a markedly narrower host range than extracellular ones, despite being more intimately tied to their host. We know relatively little about the underlying genetic changes required for a pathogen to infect a new host, though, interestingly, only a few mutations can be required for a host jump.

For example, avian influenza is only around five mutations away from being able to transmit in mammals [ 5 ], and a single amino acid change was sufficient for the human-adapted bacterium Staphylococcus aureus to become a pathogen of rabbits [ 6 ]. Genes encoding proteins specific to pathogenicity are referred to as virulence factors, which include a variety of molecules required for colonization of the host, immunoevasion and immunosuppression, scavenging nutrients within the host, and entry into and exit out of cells for intracellular pathogens.

In bacteria, virulence factors are often found in groups of genes on pathogenicity islands, which can be transferred horizontally by plasmids or other transposable elements. For example, one of the defining features of the plague bacterium Y. While acquisition of novel genes and repurposing of existing ones is essential in the evolution towards pathogenicity, a general feature during the evolution towards pathogenicity is genome reduction through the inactivation and loss of genes.

This can be primarily explained by the fact that a host represents a fairly stable and resource-rich environment where some metabolic pathways required in the environment are not necessary. Genome reduction is a general trend accompanying the evolution towards pathogenicity and is observed in Mycobacterium tuberculosis , pathogenic E. The most extreme example is leprosy M.

Another interesting tendency of many bacterial pathogens is the secondary loss of the ability to undergo genetic recombination [ 9 ]. Pathogens cause illness to their hosts through a variety of ways.

The most obvious means is through direct damage of tissues or cells during replication, generally through the production of toxins, which allows the pathogen to reach new tissues or exit the cells inside which it replicated. Bacterial toxins are among the deadliest poisons known and include famous examples such as tetanus, anthrax or botulinum toxin, known as Botox in its commercial application. However, the damage to the host is often self-inflicted through a strong or sometimes excessive immune response that indiscriminately kills infected and uninfected cells and damages host tissues.

Influenza transmits mainly through aerosols created through the sneezing and coughing it causes. Vibrio cholerae triggers a strong inflammatory response in the gut mucosa, leading to watery diarrhoea and ensuring its release in the environment and thus infection of further hosts.

Pathogens greatly vary in the severity of their symptoms from a mild inconvenience to assured death. However, this is only true in the case of strict vertical transmission such as from mother to child , where survival and transmission of host and pathogen are intimately linked. In the case of horizontal transmission, the situation is more complex and there is no straightforward way to predict the evolution of future virulence, as it will depend on a variety of factors, including the population structure of the host and the correlation between virulence and transmission [ 12 ].

A textbook example for reduction in virulence is the introduction of myxomatosis into the European rabbit population in Australia and France in and , respectively. Bd GPL, the global lineage of the amphibian fungal pathogen Batrachochytrium dendrobatidis Bd , is the only known pathogen to have extirpated entire host species.

Yet, over the three decades since its discovery, it shows no sign of evolving lower virulence. The primary reason there is little selective pressure on Bd for virulence attenuation is that it can infect a very vast host range so that the extinction of any particular host species has limited impact on its fitness.

Even worse, some host species, such as the widely introduced African clawed frog, Xenopus laevis , and the American bullfrog, Rana catesbeiana , carry the disease asymptomatically, fuelling the global Bd pandemics and limiting any short-term prospect for significant decrease in virulence [ 14 ].

These are just examples of the evolution of virulence but both illustrate that there is no simple pattern of decrease in pathogenicity with time. Apart from a few putative ancestral pathogens, including Helicobacter pylori [ 15 ], that might have co-speciated with their human host, the infectious diseases afflicting us were acquired through host jumps from other wild or domesticated animal hosts or sometimes from the wider environment.

The timing of these events and the original source remains unclear in many cases. The traditional view has been that many human pathogens emerged during the Neolithic revolution. The main arguments for an origin of human pathogens linked to agriculture are based on the proximity between traditional farmers with their livestock and the emergence of higher human population densities in stable settlements enabled by agricultural subsistence.

High population density is indeed required by some epidemic diseases which could not have maintained themselves on scattered groups of hunter-gatherers [ 12 ].

This argument, however, neglects the fact that pathogens can evolve fast. Also, while the proximity of humans and livestock is conducive to host jumps, humans transmitted more diseases to domestic animals than they acquired, with tuberculosis in particular having probably jumped from humans to cattle rather than the other way around [ 16 ]. Finally, this argument also neglects the high burden of pathogens in wild populations, including in the great apes.

Ancient direct evidence is scant for pathogens, and historical records rarely allow unambiguous attribution of described symptoms to a disease. That being said, recent progress in sequencing technology and in particular the ability to generate sequences, if not complete genomes, from ancient samples has greatly improved our understanding of the age of the major human pathogens, often leading to unexpected results.

Two of the most detrimental foodborne illnesses identified by public health systems are salmonella and norovirus. Salmonella is the cause of the most deaths associated with foodborne illness in the United States annually. Norovirus is common in healthcare and long-term care settings. It can be spread through contaminated food and surfaces, and by sick people who shed the virus in their feces, contaminate their hands and then do not wash them thoroughly.

While most infections are mild and resolve quickly, infections are more severe in the elderly, young children and those with pre-existing illnesses, who are less capable of effectively fighting off illness. Airborne pathogens are infectious microorganisms that are commonly spread through tiny airborne particles called droplet nuclei.

Typically less than 5 microns in diameter, droplet nuclei can remain suspended in the air for several hours once they are exhaled. When droplet nuclei are exhaled by someone with an infection and then inhaled by someone else, they can cause infection and disease.

Tuberculosis is an example of an infectious disease that is spread through the airborne route. Viruses that infect rodents can also be spread through the airborne route, carried through tiny dust particles.

Hantavirus, which is shed in the urine and feces of mice, can be carried through the air on dust particles and can cause an infection in humans if inhaled.

One route of pathogen transmission is through contact with contaminated surfaces. Many pathogens can survive for long periods of time — from several hours to months — outside the human body, commonly on surfaces. For example, while the influenza virus can persist on hard, nonporous surfaces such as ceramic, plastic and steel for 8—48 hours depending on the environmental conditions, 3 norovirus can persist for up to 28 days. Some bacteria such as Staphylococcus aureus can persist for up to seven months, while the spore form of Clostridium difficile can persist for up to five months.

The long survival times of common pathogens on surfaces is an issue because of the potential for transmission from surfaces to healthy people, even long after they have become contaminated. This is a concern in communal spaces such as offices, schools and athletic facilities, especially during flu season and outbreak situations, and makes effective environmental cleaning and disinfection critical to prevent the transmission of pathogens.

From health and economic perspectives, the burden resulting from illnesses caused by infectious microorganisms in the United States is huge.

Annually, foodborne pathogens such as Salmonella enterica , norovirus and Campylobacter jejuni are estimated to cause illness in one in six Americans — around 50 million, and result in , hospitalizations and 3, deaths. The burden from influenza and the common cold is also significant. Annually, people in the U. Healthcare-associated infections HAI , infections acquired while in healthcare facilities, are also a significant contributor to the burden of infectious illness.

In it was estimated that there were approximately , cases of healthcare-acquired infections recorded in U. Of these cases, around 75, died during their hospitalization. HAIs can affect any patient, but those with weakened immune systems, those undergoing intensive medical procedures, and the elderly, such as those living in long-term care facilities, are at particularly high risk.

Antibiotic resistance has been an increasing concern with regard to foodborne pathogens such as Salmonella enterica and Campylobacter jejuni. In fact, these two pathogens alone cause an estimated , antibiotic-resistant infections every year in the United States.

Norovirus Prevention: Norovirus can survive for up to 28 days on common surfaces in the environment. Only a small number of virus particles are required to cause an infection, so contaminated surfaces can contribute to the spread of the virus. Learn how effective environmental cleaning and disinfection is critical to eradicate the virus and prevent it from spreading.

In healthcare settings, transmission via contaminated surfaces is a particular concern and highlights the need to implement effective infection control protocols and practices.

Learn more about Clostridium difficile and how surface disinfection can reduce the risk of transmission. MRSA skin infections can be spread through contaminated surfaces and items used by those with infections, such as towels.

Hospital-acquired MRSA infections are more serious and can result in long, expensive hospitalization. Learn how effective disinfection is essential to prevent the spread of MRSA.

Salmonella Prevention: A salmonella infection can be spread by the consumption of contaminated water or food. Learn more about how to prevent salmonella outbreaks in your facility.

Influenza Prevention: Influenza virus particles can survive on hard surfaces for up to 48 hours after contamination, making effective environmental disinfection an important strategy to prevent transmission.

Learn more about how to prevent the spread of seasonal flu. Occupational Safety and Health Administration. Parasitic worms cause many diseases, which include :. Pathogens can spread in a variety of different ways.

For example, direct skin-to-skin contact during sex can lead to sexually transmitted infections STIs. Coughing or sneezing can cause pathogens to spread through tiny droplets in the air. These droplets can contain microorganisms, which other people breathe in. Microorganisms can also travel straight into the gut when a person consumes contaminated food or water. Bites from infected insects can also spread disease.

For example, ticks with a bacterial infection can cause Lyme disease if they bite someone, and mosquitoes with a viral infection can cause Zika virus disease. Alongside maintaining good general health, a person can take certain steps to reduce their risk of infection from pathogens. These include:. Pathogens affect all living organisms and cause illness to humans in a variety of different ways. While some pathogens cause mild problems, others can be life threatening.

Medical experts are continuing to research diseases that result from pathogens to learn more about their causes and how to treat them. Infection refers to an invasion of the body by harmful microorganisms or parasites. The severity can range from mild to fatal. Treatment depends on…. The immune system defends our body against invaders, such as viruses, bacteria, and foreign bodies. The white blood cells are a key component.

What are communicable diseases? Discover the definition of communicable diseases, the symptoms of the different types, and how to avoid them here. Airborne diseases transmit between people when droplets containing microorganisms remain suspended in the air, for example, after a person coughs…. Bacterial infections occur due to bacteria and viral infections occur due to a virus. Learn more about the difference between the two, here. What are pathogens? Medically reviewed by Jill Seladi-Schulman, Ph.

Pathogen types Diseases Transmission Prevention Takeaway Pathogens are organisms that can cause disease.