Hi Marcel: if your question is related to communication with the public (non-scientists), I think you can start by stating that 1) in true parasitism, the host is never giving or sharing willingly, the parasite is just taking, and 2) that most hosts are organisms without what we would call a 'mind', anyway, so the concept of an 'invitation' does not apply, and there are no mental states to speak of. If there is 'purposeful' sharing, the relationship might well be a mutualism, whereby something is given in return for another thing (remember, organisms can exchange food, nutrients, biomass ultimately, but also services, habitats, etc). One other thing to add perhaps is that usually parasites try and hide themselves and their effects from the host, with many possible adaptations to that goal, which only goes to prove that they are taking without any invitation.

OK, but parasitism is also used in 'human interactions', no?

Perhaps vertebrates attract/'invite' micro-organisms that boost the vertebrate immune system, like natural vaccination. Fleas are defined as nest parasites for birds, but they apparently also may have 'positive' vaccination effects at a long time scale of analysis.

In addition, so called parasites might have short-term negative effects at the individual/proximate level and longer-term positive effect at the population/ between-generation/evolutionary level. Are scales of analysis always adequately defined in publications?

I think 'parasitism' is an interpretation, and sometimes a scientifically unverified assumption in many publications.

'Only present what you see, not what you think you see'!

OK, if you extend the question to human interactions (intra-specific), then obviously some things are going to change and more detail and attention to details are needed. Then mental states begin to make a difference, since psychologically if a host 'detects' parasitism it may have a miriad of behavioural forms of getting rid of the parasite - not that another kind of organism may not have behavioural countermeasures available. But you said in the question you wanted to ignore mental states...

Regarding timescales, again the question is specifying the scientific discipline (or subdiscipline) within which you use the term parasitism, as you said. If you talk about Sociology, terminology might change relative to Community Ecology (or Interaction Ecology as some might put it), and communication problems arise. And if you consider the individual organisms your focus of study, as is usual in the definition of interspecific interactions, then what may happen in future generations is of no interest. You try and measure what is being exchanged between the two species in question, as I said earlier, whether it is biomass, specific nutrients, space, services, etc, and put it on a scale. If losses outweight gains, then you will be inclined to say this is parasitism. If losses are outweighted by gains, you will probably say it is mutualism. We might be ignorant of everything that is being exchanged, though, and ignorance it's a common problem to all Sciences. So our conclusion might be proven false in the future given more complete information. We present what we see, but we might not be seeing all.

Taking in the problem of longer timescales specifically, I think it introduces a further level on which to judge a given interaction, for sure, but it does not detract from what I described above as a sincere try at defining the nature of an interaction between two organisms from different species. It just complements it. It is the same argument: we can never be sure we are seeing the whole complex scene, and we must assume we don't see it wholly, but we must as well at least try.

Let's take the energy approach: Organisms that only take energy without giving are 'parasites'. Are avian chicks that only beg for food by manipulating parental behaviour 'parasites' during the nestling stage?

And let's change the temporal scale: When the same nestlings become helpers after they leave nest how should the nestlings be defined at a long-term basis?

I think people often do not sufficiently define the scale of analysis.

Yes, but again, now you are considering intraspecific interactions, where names and concepts attributed to interspecific interactions may not count as same. We don't call it predation if it's within the same species, we call it cannibalism. So we might perhaps not call parasitism a relationship between parents and offspring, but naming is an art, not a Science, so who knows?

Yes, who knows!

Concerning interspecific interactions, if one species rears two species (e.g. blue tit parents rear great tit and blue tit nestlings) is this a case of 'interspecific brood parasitism' or 'interspecific adoption'? Sociologists might place it in the framework of adoption, wildlife biologists in the framework of 'parasitism'. The same data, the same observations, different interpretations because of differences in background knowledge or education....

Now Sociology needs Psychology: adoption is when someone willingly chooses to be a parent of a child who is not one's offspring. Does the blue tit KNOW it is parenting a great tit? Does it have a choice? Does it benefit somehow (I doubt it, but as you say, who knows)? Again, since this is an interaction between two species, I would simply dismiss the sociological framework to adopt the ecological one - it's brood parasitism.

Perhaps the blue tits don't know they are parasitized.....

Interactions proximately involve perception and interpretation of what is perceived. This is what neurons and brains do..... A received stimulus results in a behavioural decision.... Why the decision is taken is another question...

To summarize, behavioural ecologists often use terminology applied in human social interactions, like 'parasitism', 'adoption', 'helping', 'personality profile', etc.. Why do we have to create a human-created terminology barrier between humans and other living beings? If many behavioural decisions are mainly based on 'feelings' (e.g. avoidance versus attraction versus neutral), this human vs. non-human barrier is perhaps difficult to define. Animals memorise and learn, so they 'know' to adjust behaviour to new situations....

And thanks to Milton for the constructive discussion.

Can the terminology 'parasite' be considered ambiguous?

Dear Marcel,

I am a bit amazed. You must know very well that our terminology or our second signalling system called language is imperfect. Not only the mentally expressed host parasite relationship.

I like you like paradox situations.

Dear Marcel,

What do you think?

Is our race the most dangerous parasite as not only sucking materials and energy of but also destroying Gea?

Dear Marcel,

I am sorry but your question is tricky: identifying theoretically a relationship in practice.

Dear Andras,

You work on this topic. Perhaps 'parasite', which is an interpretation, should be replaced by what is actually observed, e.g. like a plant-insect interaction or relationship?

More general as discussed elsewhere: Science often uses terminology (e.g. parasite) without providing direct evidence for it (e.g. evidence for the exact relationship between two organisms). Science stories, like those involving parasitology, are therefore often based on unverified assumptions, or not?

By the way, I have been involved in several publications using the words/terminology ectoparasite, nest parasite.....

Dear Marcel,

Sharing, giving is human. Taking without giving is sinister, where one species, the parasite (the taker), benefits at the expense of the other (the generous, the giver)! 

'Parasitism' can be identified by the lack of consensus and the lack of mutual benefits. For example the birds that 'clean-up' the skin of a big jungle animal cannot be treated as a parasit.

Dear Demetris,

But there is the importance of time scale (short-term versus long-term) and context.

Theoretical example: If the presence of the bird sitting/pecking on the big jungle animal interpreted as cooperation (cleaning for food) would reduce vigilance in the big jungle animal A, perhaps it becomes more vulnerable towards another big jungle animal B?

Parasitism is a form of symbiosis, an intimate relationship between two different species. There is a biochemical interaction between host and parasite; i.e. they recognize each other, ultimately at the molecular level, and host tissues are stimulated to react in some way. This explains why parasitism may lead to disease, but not always. It is often a life-long relationship for the parasite, which cannot survive without its host. While it is often claimed (even by definition) that a parasite must damage its host in some way (to distinguish parasitism from commensalism and mutualism), in practice this can be impossible to establish, because we know so little about most symbiotic relationships; certainly, many human parasitic infections are asymptomatic

http://parasite.org.au/para-site/introduction/introduction-essay.html

Perhaps the most simple solution would be to replace the word 'parasite(s)' by the name(s) of species you are studying, reducing the impact of unjustified interpretation? 

'Parasitism' in practice can take many forms examples of which are:

1) An able-bodied adult who refuses to do work although jobs are available depending on the social benefits of the state. The reason is solely a lazy shiftless attitude. 2) An authoritarian boss of an organization who gets “gifts” or “free dinners” from the employees. 3) A “fake” researcher who uses “subtle” ways to steal information from published work but claims that they have come from his “brain”. 4) A “liar” who has the habit of telling others that s/he is poor & needs help but later on is found to be stacking a huge fortune. 5) A university “dishonest” professor who reached the title upon “utilizing” the efforts of the students in a sneaky way.

ALL THE ABOVE 5 ARE PARASITES.

Dear Dr. Nizar Matar,

All of your examples belong to the area of ethics. It seems to me that being a parasite in sociological sense must be a very remunerative way of life. I meet these parasite types every day.

Dear Marcel,

There can be distinguished two main types of biological parasites: parasites and parasitoids.

Parasites generally do not kill even their individual host but weaken host individuals and populations contributing to a kind of selection. Parasitoids generally kill their individual hosts but do not destroy host populations. Also parasitods start an evolutionary development at population level from their own and that of the host population’s point of view.

Example:

Protocalliphora larvae living on bird nestlings but not directly killing nestlings versus Nasonia living on Protocalliphora larvae and killing them. Would Nasonia be considered beneficial for bird nestlings when they can control the Protocalliphora population?

Do Protocalliphora contribute to the selection of more resistent birds reproducing in next generations? Why Protocalliphora would be defined as a parasite when it would biologically strengthen, not biologically weaken, bird populations in next genrations? 

Dear Francesca,

There is huge diversity in flies and they also differ a lot in personality profiles. Please do no fall into the trap of discrimination based on judgement that is based on a single observation

A statement supposedly from Prof. Jason Read got from Internet – "People who dismiss the unemployed and dependent as ‘parasites’ fail to understand economics and parasitism. A successful parasite is one that is not recognized by its host, one that can make its host work for it without appearing as a burden. Such is the ruling class in a capitalist society.”

Dear Marcel,

1 Nestlings, Protocalliphora and Nasonia relationship is much more complex than what you mentioned above.

2 I have not studied this relationship at all.

3 Regarding your comment Nasonia may have a beneficial impact on nestlings.

4 A lot of factors determine the consequences of the nestling Protocalliphore connection. It is difficult to forecast strengthening impacts of Protocallifora for future bird populations. I think tolerance of various bird species to Protocalliphora may be different.

Dear Andras,

Your claims 1 and 2 are contradictory, or not? How do you know these ecological relationships are very comple without investigating them? Can findings from system A be applied to system B?

In addition to the 5 examples I gave on page 3 of this thread, I was reminded of another example of a "social" parasite by a story which occurred few days ago. It is about some one who works in an organization by virtue of affiliations & relations but not on the basis of qualifying for the job itself. To keep his well-paid job "which is threatened by the presence of more qualified persons under his command", he opted for spying & writing negative reports about the organization's staff to the top managers. The top managers fell under the illusion of knowing the details & of being updated with information. This guy depicts parasitism in its most ugly picture since it is like survival but by walking over the skulls of the human beings.  

Virtual scenario about collaboration versus parasitism versus ???:

Person A talks negatively to person B about person C. Person A wants to get a response from person B to have an idea how person B thinks about person C. Then person A transmits the response of person B to person C.

Without mind reading, how will person B identify his/her social position towards persons A and C from a short-term and long-term point of view?

Should by definition parasites always be physically attached to hosts, or not? Based on the examples provided, the key is apparently draining energy from others in an unbalanced way, what then what about the definition of predation versus the definition of parasitism versus the definition of exploitation versus the definition of .....?

Dear Francesca,

When I give something to someone, I do not have in mind what I will or wish to receive back.

Cheers

Dear Francesca,

As an example:

If humans give blood, they do not know to whom they give it, and of course they expect not getting blood back. This also applies when you finance an association and you do not know who will receive the support, or when you give money to poor people in the street you don't know, etc.....

Dear Marcel and Francesca,

Giving for the sake of giving is human, is an act of goodness; taking for the sake of taking is mean and irresponsible, and, therefore, vicious! 

Only information Francesca, not more than that.

Many thanks for all these examples

Dear Francesca, I think you have  covered all the aspects of parasitism!

I agree with you and I have to search in order to find new examples, after your post!

:)

Just to add that every kind of usury is the worst case of parasitism: Look the IMF project and its parties:

http://www.keeptalkinggreece.com/2012/12/20/christmas-2012-gala-dinner-imf-knows-how-to-party-with-caviar-oysters-truffle/

Who do you think that really paid that party? Of course Greece!

"That might seem like a lot, but look at it this way: Greece is expected to cough up $899 million in interest payments to the IMF next year."

Parasites are microorganisms that live off of other organisms, or hosts, to survive. Some parasites don’t affect the host. Others grow, reproduce, or give off toxins that make the host sick resulting in a parasitic infection.

Parasitic infections are a large problem in tropical and subtropical regions. Malaria is one of the deadliest diseases caused by a parasite. 

Types of Parasitic Infections

Parasitic infections can be caused by three types of organisms: protozoa, helminths, and ectoparasites. Protozoa are single-celled organisms that can live and multiply inside of humans. Some infections caused by protozoa include giardiasis. This is a serious infection that can happen from drinking water infected with the protozoa. Helminths are multi-celled organisms that can live alone or in humans. These are commonly known as worms and include flatworms, tapeworms, ringworms, and roundworms. Ectoparasites are multi-celled organisms that live in or feed off of the skin of humans, such as mosquitos, fleas, ticks, and mites.

http://www.healthline.com/health/parasitic-infections#Types2

Dear Shafig,

Thus in a vertebrate-parasite system you assume somehow that the parasite is always substantially physically smaller in body size than the vertebrate. By contrast, in the social sciences illustrated by Francesca, it is the behaviour, not the morhology, that defines what a parasite is, or not? Is there a definition that combines the two, e.g. related to unequal energy transfer, but then how to make the difference with interactions involving predation?

The interesting is that people/animals are born with teeth to consume other living beings. You cannot blame living beings to live?

And what about a whale? Is this species an eco-system parasite because it consumes so much krill? How many living beings use the whale as habitat? Billions?

Is an extreme form of parental care a form of (social) parasitism, e.g. parents not stopping sharing and loving and therefore not allowing offspring to become independent?

Is a parasite defined at the individual level or population level?

Example:

Bacterium X will be defined as parasitic when a huge population will make a vertebrate sick (e.g. called infection) versus the same bacterium X will not be defined as parasitic when numbers within an individual vertebrate are undetectably small, e.g. when they are part of a huge community of many different bacteria species?

Why is a species called 'a parasite' when it is established that the species has life cycle moments in an unparasitic state?

What is the minimum fraction of a whole life cycle of an individual required to be called parasite?