Bacteria, gut organisms linked to health, autism, schizophrenia, depression, diabetes, allergies and obesity
They may have the power to alter your thoughts, your behaviour, how well you feel or influence why you can't shed those unwanted kilos.
They outnumber us 100 trillion to one and they know you intimately.
The number of bacteria and other organisms in the human gut, known collectively as microbiota, are thought to be approximately 10 times more numerous than the entire number of cells in the body and scientists are only just starting to unlock the secrets of how they affect us.
As the world of gut bacteria and other microscopic organisms in our bodies begins to be explored it turns out the organisms are weirder than anyone thought possible.
They have been described as an organ of the body in their own right, and could hold the key to preventing or treating obesity, asthma, diabetes, autism or schizophrenia.
Some bacteria's impact is undisputed. Escheria coli, better known as E.coli, is known to cause food poisoning and is largely transmitted when people inadvertently swallow faeces that contain it.
But the jury is still out for how other microbiota effect people.
While bacteria have evolved to live in humans, the human body has also evolved to cater to the bacteria and other organisms living within us.
In some cases it almost seems that humans appear to be willing zombies, catering to the whims of the visitors in our intestines.
Mice unafraid of cats
In animal studies the evidence is intriguing.
If mice are treated with an intestinal parasite called toxoplasma the rodents lose their fear of cats.
Toxoplasma needs to reproduce inside the guts of felines, and these tiny organisms somehow can alter the mice's behaviour.
Normally fearful of cat smell, they become attracted to cats, even displaying the same behaviours if they are treated to remove the parasites, suggesting they permanently change the animals' brains.
Some humans also have toxoplasma in their gut and studies are underway to see whether they can influence our actions, too.
In people toxoplasma have been linked to schizophrenia and bipolar disorder, although the jury is still out on whether the parasite is a symptom or a cause of the problems.
The insect world is not immune to the influence of micro-organisms, either.
Jungle-dwelling turtle ants are known to act oddly when a particular nematode enters them.
The ants' rear turns red, and the creatures go to tree tops where they wave their abdomen around.
Birds, apparently attracted to what they think is a shiny fruit, eat the ants, leaving the nematodes to complete their life cycle, which must occur inside birds.
Cause of 'Western diseases'?
Professor Charles Mackay from Sydney University is a medical researcher with 35 years' experience and he believes gut microbiota may hold the key to unlocking most so-called Western lifestyle diseases.
"When you look at almost any condition that exists now that didn't exist 40 or 50 years ago, or was much less common 40 or 50 years ago, there is a good chance that it is relating to the actions of the gut microbiota," Professor Mackay said.
"I would include in that list food allergies, asthma, type one and type two diabetes, obesity and possibly autism and a few other things.
"That is what I think is likely although some of that is not yet fully established, but that is how things are shaping up," he said.
He said some cancers, including colon cancer, may also be impacted by people's intestinal flora, and conditions such as inflammatory bowel disease.
Professor Mackay said the course of study could have a bigger impact on human health than any other medical breakthrough for 30 or 40 years.
"This is a revolution in human medicine," he said.
He said the study is still an evolving field, but one that is causing an explosion in interest.
Professor Mackay explained the rationale behind how bacteria could be impacting on so many different parts of the human body.
"There are these trillions of bacteria in our gut that are pouring out molecules because they serve a function in our gut to digest fibre and do a whole lot of other things, and produce metabolites.
"These metabolites they can enter the blood and go all over the body.
"A bacteria that might infect your gut could be producing some molecule, maybe a common one, or a less common one, that has some fundamental role on a distal cell, maybe a nerve cell or another type of cell for another disease that is profoundly affecting its function, or adversely affecting its function," he said.
Trouble losing weight? Some bacteria like sugars and other like fats.
Professor Mackay said such links are interesting, but no proof has yet been found to show unhealthy cravings are determined by gut bacteria.
A major US study known as the Human Microbiome Project has tried to identify micro-organisms associated with both healthy and unhealthy people.
The study has already shown that microbes contribute more genes responsible for people's survival than humans' own genes, and that there are more than 10,000 microbial species in the human ecosystem.
While bacteria can negatively impact on the body, possibly even controlling thoughts, the good news is that they also offer ways to help people, and studies have shown that bacteria levels can be manipulated to make people healthier.
PhD student at Queensland University Megan Rossi conducted her thesis into links between gut bacteria and kidney disease.
She found that loading up kidney disease patients with good bacteria, known as pro-biotics, and healthy food for the bacteria to live in, known as pre-biotics, helped the patients.
"Compared to placebos the pre and pro-biotics were able to inhibit the production of those harmful toxins, resulting in a significant decrease in their blood concentration," Ms Rossi said earlier this year.
"Further none of the patients reported any adverse side-effects, nor did it impact on the quality of their lives."
But before you go loading up with pro-biotic capsules from the chemist or good bacteria drinks for sale in supermarkets, beware that they are only part of the solution.
Professor Mackay said the pre-biotics stage was crucial, because having loads of good bacteria is not helpful if the bacteria do not have the fibre or other healthy things they need to multiply.
Also, many of the bacteria identified in the human body have not been able to be grown in a laboratory, so some that may be crucial for good health cannot yet be replicated in a pill.
Although it may seem unsavoury, having faeces from healthy people loaded with good bacteria transplanted directly into the intestines of sick patients is increasingly being seen as a useful tool by doctors.
Known as faecal microbiota transplantation it has been shown to effectively treat patients suffering from an infection known as Clostridium difficile, which has been tough to treat using conventional methods.
C. difficile has been linked to thousands of deaths per year in the USA so any new method of dealing with it has been welcomed.
But could unhealthy, fat, diabetic or autistic people or those with food allergies one day be cured by stool transplants from healthy people?
"Maybe faecal transplants will be useful for a wide range of diseases but I think it won't be long before we figure out the right combinations of bacteria that can achieve the same effect, lets see, that would be more palatable, and controlled than faecal transplants," Professor Mackay said.
"Regardless, these types of new approaches to human diseases are exciting and could change medical practice," he said.