If there's one thing emblazoned across every science writer's desk, it's the eternal quote "correlation is not causation". As if to prove the eternal validity of this quote, we have a study from Sweden that tracked oral hygiene and cancer deaths over a period of 24 years for 1,400 individuals. Lo and behold, there's a link between plaque levels and cancer mortality.
Top image: Smabs Sputzer/Flickr.
Even the authors are quick to note that they're not saying that plaque causes cancer:
"Our study hypothesis was confirmed by the finding that poor [mouth] hygiene, as reflected in the amount of dental plaque, was associated with increased cancer mortality. Further studies are required to determine whether there is any causal element in the observed association."
However, the way that failing to brush your teeth could potentially be linked to cancer is because of the nasty things which can grow in your mouth. Uncontrolled biofilm buildup causes pockets of microbial cells, toxins and enzymes, which carry a high bacterial load, and could enter the blood stream, which could lead to systemic problems.
Alternatively, there are a bucket load of self-inflicted causes of various types of cancer out there, some of which are doubtless linked to overall poor bodily hygiene and lack of medical treatment. It's also entirely possible that people less likely to brush their teeth are also more likely to do Y, which in turn leads to cancer X.
Also, I wouldn't worry about it too much — while there was a 79% increase in the chance of an early death, only 58 out of the 1,390 participants in the study passed on early.