Diseases such as diabetes could be caused by children being too clean, researchers have suggested.
It is thought to be important for children to be exposed to germs
Scientists from the Scripps Research Institute in California, say being exposed to too few germs means the immune system is not stimulated enough.
Writing in Cell, they say this means the body then has too few immune cells.
However, other studies have said there are enough cells, but because they have not "met" germs, they are not fully equipped to fight future infections.
Both theories are attempting to explain why in Type 1 diabetes, T cells turn on the body, attacking and kill beta cells in the pancreas, the body's source of insulin.
The Scripps researchers suggest that the lack of exposure to bacteria and viruses means the immune system does not work hard enough, creating a condition known as lymphopenia, where there is a reduction in the number of T cells in the body.
They suggest people with autoimmune diseases like Type 1 diabetes, lupus, and rheumatoid arthritis often have low T cell numbers.
They say it might be possible to reduce the chance that people will become ill by "priming" the immune system with germs.
The researchers studied mice living in an extremely sterile environment which had been genetically modified so they were prone to developing diabetes.
But when the mice were exposed to bacteria, their T cell count increased and curtailed the development of diabetes in the mice.
They say the protection against diabetes resulted from exposure to these pathogens because it kept the body full of immune cells.
Nora Sarvetnick, professor of immunology at Scripps, who led the research, said: "Autoimmunity has [traditionally] been considered a condition of too much stimulation.
"What we are seeing is that it is a condition of too little stimulation."
However other researchers have cast doubt on the American research.
Dr Mark Peakman, an immunobiologist from King¿s and St. Thomas' School of Medicine in London, who has also looked at why people develop diabetes, told BBC News Online the mice were living in a much more sterile environment than people ever would, so the results may not translate to humans.
He added: "It is also not commonly accepted that people with autoimmune diseases have low T cell counts. To say that is scaremongering."
Dr Patricia McKinney, of the University of Leeds added other research had suggested childhood exposure to bacteria and viruses protected against diabetes.
She said: "We know that children who go to day nurseries in their early life are less likely to develop Type 1 diabetes."