They are transferred from one person to another explained the University of North Carolina's Dr. William Vann.
"We understand how children get the bacteria, they get them from the primary caretaker, most of the time the Mom," Dr. Vann said.
All children are susceptible, but research shows there is a window which is especially critical in little ones, from six to 36 months of age.
So what is a parent or caregiver to do, aside from put a child in a bubble?
"You hear stories about, well, you shouldn't share utensils, you shouldn't taste your baby's food, and those things are true, but the reality is, you can't break the chain of transmission," said Dr. Vann.
Still, parents and caregivers can help reduce the risk by taking care of their own mouth, which keeps the number of decay causing germs low so that fewer are passed on.
"As soon as the teeth come in, the bacteria are going to find those teeth," explained Dr. Vann.
But that doesn't mean cavities are inevitable.
Good oral hygiene, healthy eating habits, and regular trips to the dentist can keep the bad bugs at bay.
Genetics also play a role, making some children more susceptible and others more resilient to the cavity-causing bacteria.
The American Academy of Pediatric Dentistry recommends youngsters have their first dental check-up by the time they turn a year old.