Aside from the obvious issues of bad breath, pain, abscessed teeth, tooth loss and gum disease, because tooth decay is caused by bacteria - scientists have actually linked it to several other health issues.
These can include heart attack, stroke, diabetes, preterm birth and low birth weight (in pregnant women), repiratory disease, osteoporosis and digestive disorders.
Most dentists agree that tooth decay is primarily an oral problem however, it is often indicative of other problems as well. Consider also that as the body fights the bacteria in the mouth, the entire immune system becomes more compromised. As pain develops, the body weakens. Because our bodies are fully functioning machines, nothing really works independently of anything else.