Any first-grader can tell you that if you don’t brush your teeth, creepy sugar “bugs” will take over and cause cavities.

But not all of the bugs, or microbes, in the mouth actually affect oral health. And not all of them are bad.

A vast and diverse community of microscopic organisms — including viruses, bacteria, fungi and protozoa — thrive in the mouth, scientists are learning. Some of them may play important roles in preserving health and causing diseases, says Dr. David Relman, an associate professor of microbiology at Stanford University.

“We already know so much about so many disease-causing agents that it tends to give you the feeling that there is no reason to be going out and trying to find others,” he says.

But the causes of many diseases remain unknown — and the mouth appears to be a good place to look for clues.

“We are increasingly becoming aware of the connection between the oral cavity and the body,” says Donna Mager, a researcher at the Forsyth Institute, an independent research center in Boston. “Whatever grows in the oral cavity could have an impact on what happens to the body.”

Research by Relman and Mager is helping to advance this notion.

In his lab, Relman samples the region in the mouth called the subgingival crevice, the deep space between the gum and tooth. So far, scientists have found more than 500 bacterial strains or microbes in this area, and Relman has identified 37 others. Other mouth microbes have yet to be identified, he believes.

Ultimately, researchers want to better understand the patterns and prevalence of particular microbes and what those patterns suggest about a person’s health.

“I think it’s fair to say that the vast majority of members of the oral microbial flora are not disease-causing agents. They may be necessary or important for maintaining health,” he says. “Some small subset may be involved in disease.”

Gum disease, in which gums become inflamed, red and swollen because of plaque deposits on teeth, is probably caused by several organisms, but no one has identified all the microbes responsible for this common disorder.

Some microbes found in the mouth may even contribute to disease in other parts of the body, such as the heart, although those links are still tenuous.

Mager is trying to identify possibly precancerous changes in the mouth’s bacteria colony. About half of all people diagnosed with oral cancer die within five years because the disease is detected at advanced stages. But if scientists could find clues that disease could develop — based on changes in bacteria — they might be able to intervene earlier, she says.

“We are tracking the changes in those colonization patterns,” says Mager, whose research is in collaboration with the Dana Farber Cancer Institute. “Hopefully, we will be able to predict which people will develop cancer.”