If parasites are organisms that feed on other living organisms, then aren't all pathogens parasites?

I've emphasised on 'feed' because I guess the pathogens just eat us (i.e. take nutrients to reproduce and sustain their lives), right?

But then there's another question.
If, pathogens need us (a host) to survive, then why do they eventually kill us? I mean, why did they evolve to become an organism that exploits their food to the extent that even they, themselves die? Isn't it disadvantageous for pathogens to kill their hosts, because they will eventually die?
Or I'm totally wrong, do those pathogen sustain their lives even when host has died?
Or perhaps they don't care if they, as individual organisms die, because they can survive as an species by infecting other.

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    $\begingroup$ Welcome to Biology.SE. Parasites usually don't kill their host, parasitoids do. Parasites don't necessarily feed on other living organisms. There are two (or three) definitions of parasitism causing the debates under the current posts. I don't really understand the relationship between the question on the first line (which is a semantic issue) and the question(s) in the last paragraph. $\endgroup$ – Remi.b Aug 30 '15 at 6:59
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    $\begingroup$ The post "Why do parasites sometimes kill their host?" will probably answer your last paragraph. $\endgroup$ – Remi.b Aug 30 '15 at 6:59

Not all pathogens are parasites. Many opportunistic infections can be caused by organisms that are normally commensal or even mutualistic.

For example, this paper describes how multiple bacteria species can be pathogenic as well as mutualistic.

Despite its generally innocuous nature, over the past 20 years S. epidermidis has emerged as a frequent cause of nosocomial infections. Several extrinsic factors contribute to the conversion of S. epidermidis from a member of the resident microflora to an infectious agent. The bacteria primarily infect compromised patients including drug abusers, those on immunosuppressive therapy, patients with acquired immune deficiency syndrome (AIDS), premature neonates and patients with an indwelling device.

... Recent studies can be interpreted to suggest that S. epidermidis is a mutualistic organism, much like the bacteria of the gut. Many strains of S. epidermidis produce lantibiotics, which are lanthionine-containing antibacterial peptides, also known as bacteriocins (Fig. 3). Among the several identified bacteriocins are epidermin, epilancin K7, epilancin 15X, Pep5 and staphylococcin 1580.37–39 Additional antimicrobial peptides on the surface of the skin have recently been identified as originating from S. epidermidis.

... The removal of S. epidermidis (i.e. through overuse of topical antibiotics) may be detrimental to the host for two reasons. Firstly, removing S. epidermidis eliminates the bacterium’s endogenous antimicrobial peptides, allowing potentially pathogenic organisms to colonize the skin more effectively. Secondly, without bacterial priming of the skin, the host may be less efficient in warding off infection.

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    $\begingroup$ When they are being pathogenic, they are parasitic. Parasitism may not be exhibited all the time. You may say that they exhibit facultative or conditional parasitism. But they cannot be pathogenic and non-parasitic. The only case where that could be true is when the microbe itself dies but also causes harm to the host (like in case of endotoxins). $\endgroup$ – WYSIWYG Aug 29 '15 at 15:25
  • $\begingroup$ @WYSIWYG Your statement is absolutely correct. I was intending to show that parasitic (or mutualistic) relationships are not fixed, and can change from time to time. $\endgroup$ – March Ho Aug 30 '15 at 0:41

Not only active parasitism by pathogens, but any kind of interaction that leads to advantage of one species while causing disadvantage to the other species is considered a parasitic interaction. In this post, Remi has explained why even Batesian mimicry can be considered a parasitic interaction. The only case where a microbe could be pathogenic but non-parasitic is when the microbe itself dies but also causes harm to the host (like in case of endotoxins released after death of the bacterial cell).

Now, as you pointed out, killing the host is like wasting the food but killing of the host is an inevitable consequence of the infection. In fact not all successful pathogens kill their hosts (such as influenza; people die of influenza but it generally has low mortality rate). You can think of this as an optimization problem. The pathogen has to maximize the growth with a constraint on available resources. If the host population size is very large and the pathogen has a decent infectivity, then killing the host would not matter as long as the host population remains more or less stable.

Some pathogenic microbes can even grow for a while inside the corpse. Most of these pathogens are of the opportunistic types. They would thrive on the body of the host better when the immune system has collapsed (which happens at the time of death). These microbes also assist in the decaying of the body.

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    $\begingroup$ From an Immunology standpoint parasites are viewed as worms or protists. The discipline of Parasitology focuses on unicellular and multicellular Eukaryotic organisms, where as Prokaryotic organisms are the purview of Bacteriology. While the strict definition of the word would allow for bacteria to be considered parasitic, one could also make the argument that a fetus can also be parasitic, especially if the result of the pregnancy is gestational diabetes or the death of the mother in childbirth. $\endgroup$ – AMR Aug 29 '15 at 15:57
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    $\begingroup$ @AMR from an ecological standpoint, parasitism is any interaction that has a +/- effect. Did you have a look at the linked post? $\endgroup$ – WYSIWYG Aug 29 '15 at 16:49
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    $\begingroup$ Yes, however as the question is asked from the perspective of a pathogen, then the definitions of immunology, parasitologist, an bacteriology have more relevance and have a more specific definition of terms. $\endgroup$ – AMR Aug 29 '15 at 17:18
  • $\begingroup$ Fetus, parasitic? I don't know the definition, but my logic says that an organism using another organism of the same species can't be parasitism.. It was a consequence of an effort to continue a particular species, using the organism of same species. It was not effort made by another species, to continue living, by using another one... But this is not the case with microbes. A worm is a parasite because it's multicellular, and a microscopic pathogen is not, because it's unicellular, that doesn't seem reasonable! $\endgroup$ – Perceptioner Aug 30 '15 at 5:05
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    $\begingroup$ Look at my comments to Remi.b's answer below. Parasitic disease are not considered bacteriological in nature, though bacteria can act parasitically. The definition of parasite and parasitism likely predates the discipline of immunology, so I would hardly say that the immunological term is obsolete. Also as I pointed out, there is an important distinction as bacterial infections have different treatment protocols that those of infections derived from Eukaryotic organisms. Modern antibiotics actually exploit those differences to kill bacteria while leaving the eukaryotic host relatively unharmed $\endgroup$ – AMR Aug 30 '15 at 8:02

The question is a matter of definitions.

  1. A parasite is an agent that causes harm to another agent

  2. A pathogen is an agent that causes disease to another agent.

  3. A disease is bad.

Therefore, all pathogens are necessarily parasites. Note that some definitions of parasites, imply that the parasite benefits from the relationship. In which case, then the above sentence (in bold) is true only if all pathogens benefit from being pathogenic.

However, it is important that classifying all living things into one or another category is not feasible. organisms from many species can sometimes be called mutualist, commensal and parasitic (to another agent) depending on the ressources and the life-cycle. For example, "symbiotic" algea are mutualistic when there is light but are parasitic in the absence of light.

  • $\begingroup$ As I mentioned in WYSIWYG's answer, from an Immunology perspective, while pathogenic bacteria act parasitically, the classification of an organism being a parasite is limited to Eukaryotic protists or multicellular organisms. The distinction is important as there are much different protocols for treating infections by Eukaryotic organisms than there are for Prokaryotic ones. $\endgroup$ – AMR Aug 30 '15 at 6:45
  • $\begingroup$ The Wikipedia article on parasitic disease goes into more detail. en.wikipedia.org/wiki/Parasitic_disease $\endgroup$ – AMR Aug 30 '15 at 6:49
  • $\begingroup$ I wasn't aware of this semantic gap between ecology and immunology. Reading fully the OP's question (I eventually just read the title before :)) I realize that there are a bunch of misconceptions. parasite vs parasitoid, species survival thing, fate of the host, etc... I am too tired to write a complete answer on that now. And I guess there's the issue of the several definitions of parasitism. $\endgroup$ – Remi.b Aug 30 '15 at 6:55

While all pathogens act parasitically, a definitional distinction is made between Eukaryotic parasites; protists, worms, and ectoparasites such as lice, and pathogens such as bacteria, viruses, and even eukaryotic fungi.

The Center For Disease Control's resource on Parasites states:

A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host. There are three main classes of parasites that can cause disease in humans: protozoa, helminths, and ectoparasites.

The World Health Organization fact sheet on Antimicrobial Resistance refers to parasites separately from Bacteria, Viruses, or Fungi:

Antimicrobial resistance threatens the effective prevention and treatment of an ever-increasing range of infections caused by bacteria, parasites, viruses and fungi.

The American College of Physicians makes the same distinction on its page describing the specialty of Infectious Diseases:

ID specialists have expertise in infections of the sinuses, heart, brain, lungs, urinary tract, bowel, bones and pelvic organs. Their extensive training focuses on all kinds of infections, including those caused by bacteria, viruses, fungi and parasites. Many ID physicians specialize in treating patients with infections due to human immunodeficiency virus (HIV), the cause of AIDS.

Even the Wikipedia page on Parasitic Diseases points out the distinction:


Although organisms such as bacteria function as parasites, the usage of the term "parasitic disease" is usually more restricted. The three main types of organisms causing these conditions are protozoa (causing protozoan infection), helminths (helminthiasis), and ectoparasites.(1) Protozoa and helminths are usually endoparasites (usually living inside the body of the host), while ectoparasites usually live on the surface of the host. Occasionally the definition of "parasitic disease" is restricted to diseases due to endoparasites.(2)

In my opinion this is an important distinction to make. The treatment protocol for prokaryotic organisms and eukaryotic organisms are different. Evolutionarily, eukaryotic organisms are far closer to us than prokaryotes, and so strategies such as disrupting the ability of a ribosome to translate proteins, a method that some classes of antibiotics employ, would not only affect the protist or worm, it would more than likely affect our own cells ability to produce proteins. In bacteria, the ribosomal subunit is different enough that it can be targeted by medicines, yet similar enough that we can use them in recombinant DNA technology to reproduce mammalian genes. The closer we get evolutionarily to the pathogen, the more alike their molecular machinery is to our own.

Even our own immune system makes a distinction. Leukocytes such as eosinophils are usually only raised to combat parasitic infections. Organisms that are medically termed as parasites also pose a problem in that they are often larger, protists, or much larger, worms, and our phagocytic cells such as macrophages and neutrophils are unable to phagocytose them as a way of clearing an infection.

From your question, you ask

If parasites are organisms that feed on other living organisms, then aren't all pathogens parasites?

I've emphasised on 'feed' because I guess the pathogens just eat us (i.e. take nutrients to reproduce and sustain their lives), right?

There are a host of organisms that live in and on the human body, that feed and reproduce in us and on us that do not do us any harm, known as commensals, and might even be symbiotic, gut flora. However commensals and even symbionts can do harm if they gain access to areas of the body other than their niche locales.

In fact, immunologists often view commensal organisms as part of the bodies innate immune defenses against pathogens. Because they take up space and use resources that would otherwise be used by pathogenic organisms, then serve to limit the ability of pathogens to reproduce and to gain access to tissue that would be vulnerable to the pathogen.

Commensal organisms can become opportunistically pathogenic if they gain access to sterile environments within the body that have not evolved to live "in harmony" with these organisms. An example of this is peritonitis. We can live our lives perfectly fine when our intestinal bacteria remain within the intestines and the colon, however if those tissue are damaged and perforated, these organisms that posed no risk to our health, and actually probably provide a benefit can lead to serious infection and possible death.

There are also commensal Eukaryotes, such as mites and fungi that either do no harm or are kept in check by a properly functioning immune system.

You also ask:

But then there's another question. If, pathogens need us (a host) to survive, then why do they eventually kill us? I mean, why did they evolve to become an organism that exploits their food to the extent that even they, themselves die? Isn't it disadvantageous for pathogens to kill their hosts, because they will eventually die? Or I'm totally wrong, do those pathogen sustain their lives even when host has died? Or perhaps they don't care if they, as individual organisms die, because they can survive as an species by infecting other.

I think where you might be going wrong here is that you are thinking in terms of sanitary and clean forms of death and burial that is a social construct of human primates. Through much of the evolutionary history of multicellular organisms, when organisms died, they died in place and decomposed. This decomposition allowed the pathogenic organisms a route to go on and spread to other organisms. Other living organism came in contact with the decomposition and the pathogen had a way of spreading.

Also many bacteria and viruses can survive outside of their host for extended periods of time, so they in effect are dormant in the environment until they come in contact with a new host that they have the opportunity to infect. In this case, the most important thing for them is to escape the host until another suitable host comes along that can sustain an infection.

Though there are pathogens that have evolved the ability to circumvent our social norms. The body of a victim of the Ebola virus is highly contagious and can infect those that ritualistically tend to the body for burial. The WHO has developed guidelines to try and respect the cultural customs of burial while also preventing the spread of the disease. You could also think of Cholera. Vibrio cholera reproduces extremely rapidly in the host, and the toxins that it produces cause the host to expel the pathogen at a rate that can kill. This serves cholera as it can then go on to infect a new host.

You also have to remember that there are pathogens that are pathogenic in one organism but commensal in others. The parasite that causes malaria is commensal in mosquitos. The vector for Ebola is thought to be bats, but bats do not succumb to the disease. So for pathogens like this, the strategy would be to find a reservoir vector, then find a host where they can replicate, then escape that host either to reinfect another host or find another carrier where it can survive commensally.

  • $\begingroup$ The sites that you have mentioned and the classification that they make is more like a layman classification. It is not in strict sense based on any particular convention. These sites may be of credible institutions but the sites themselves are not standard sources of reference. Plus, I again emphasize, parasitism is an ecological relationship and not a classification of organisms. Phylogenetics is there for the classification. $\endgroup$ – WYSIWYG Aug 30 '15 at 18:59
  • $\begingroup$ The audience for ACP is prospective physicians.... It is how Janeway Immunobiology, the textbook recommended by many graduate Immunology and medical schools refer to it. It is how Parasitologists refer to it. It is how Bacteriologists refer to it. You seem to be the only person that I can find that has a problem with it. If I go to a doctor and they diagnose me as having a parasite, they will never be referring to viruses or bacteria... If my dog' vet tells me he has a parasite, I guarantee that 100% they are referring to a protist or worm, not bacteria or a virus, not even fleas or ticks. $\endgroup$ – AMR Aug 30 '15 at 19:15
  • $\begingroup$ Yes because your doctor or vet is supposed to interact with laymen and not give scientific discourses to their patients. The very definition of a parasite as you quoted is general. If parasite means only certain organisms then the definition should say it explicitly. You seem to justify a wrong usage simple because a lot of people do it. You also seem to hold a view that textbooks are infallible. $\endgroup$ – WYSIWYG Aug 30 '15 at 19:30
  • $\begingroup$ The majority of professionals that are responsible for the definitions have made that distinction. You have three distinct branches of study, Parasitology, Bacteriology, and Virology. The Immunology professor I had makes that distinction, and she corrected me when I did not use the term renature to describe proteins refolding into their proper conformation after denaturation. The people whose careers, research, and livelihoods are in these fields make the distinction. If I was quoting one obscure source, I could see being doubtful, but you are being like The Black Knight in The Holy Grail. $\endgroup$ – AMR Aug 30 '15 at 21:16

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