Alzheimer's disease, rheumatoid arthritis and a form of pneumonia could all begin in the gums.
Bacteria from infected gums have been found in the brains of Alzheimer's patients, and scientists have started to understand how it can move through the body from the infected gums and into the brain.
They suspect the bacteria from gum disease, or periodontitis, could also be a cause of rheumatoid arthritis and aspiration pneumonia, a lung infection from inhaling food or saliva.
The gum bacteria, Porphyromonas gingivalis, has been found in the brains of Alzheimer's patients, say researchers from the University of Louisville School of Dentistry, but wasn't in the brain samples from people who never suffered from the disease.
The P gingivalis bacteria affects the gums of around 20 per cent of people and can be harmful if it progresses to trigger an immune system response that causes inflammation, leading to swelling and bleeding in the gum and the erosion of gum tissue, by which time it is diagnosed as periodontitis.