Scientists have claimed that a growing obsession with cleanliness could be making people depressed.
They believe that eliminating bacteria and viruses has actually made our immune systems weaker, and this in turn has affected the functioning of our brains.
Scientists have long blamed our overly sterile environments for increases in asthma and allergies. They say some bacteria are necessary for bolstering our immune systems, and without them our bodies over-react to dust and pollen, resulting in an allergy.
But now researchers believe that this over-reaction may also impair the brain's ability to produce certain chemicals that make us happy - such as serotonin - and this leads to depression.
Researchers in Atlanta, Georgia, have studied how this over-reaction, or inflammation, affects the brain by recruiting a group of 27 patients taking drugs to treat hepatitis C - which causes similar reactions.
They believe certain reactions may affect the brain's ability to produce certain chemicals including serotonin, known as the "happy hormone".
"We believe the immune system is causing depression," the Daily Mail quoted Dr Andrew Miller, one of the scientists, as saying.
"As people develop and grow up, their immune system develops. If they are exposed to more bacteria and parasites, they are able to better control the inflammation.
"Nowadays people's environment is much cleaner and hygienic so our immune system never really learns how to deal with infectious agents. We are overactive because our immune system has not been trained," added Miller.
The study has been published in the journal Molecular Pyschiatry.