Investigators found that brain tissue from patients who suffered from the mental illness was infected with bugs that cause gum disease.
"The bacteria could be a trigger that sets off a chain reaction in people predisposed to dementia," said dentistry professor Stjohn Crean. "People should pay particular attention to brushing and visiting the dentist to reduce the amount of bugs in their mouths -- especially those with a history of dementia in their family."
The study found that the bug, Porphyromonas gingivalis, triggers an immune response in the brain that destroys neurons and brings about memory loss and confusion.
The bacteria commonly enters the bloodstream during dental treatments such as fillings and root canal surgery and while eating.
In their findings the team noted that 10 people who suffered from dementia had the bug present in their brain tissue, while 10 others who didn't suffer form the disease did not.
In addition, a previous study on 5500 elderly people found that those who brushed their teeth less than once a day were 65 percent more likely to develop dementia than those who brushed three times a day.