Our aversion to contamination by pathogens is so strong that most evolutionary psychologists believe that's how our sense of disgust evolved: for example, to prevent us from coming into contact with parasite-infested rotten meat, feces, and so on. And clearly any visible sign of infectious disease -- smallpox, leprosy, even a herpes blister -- is enough to make us recoil a bit. These programmed responses make perfect sense: our avoidance of pathogens contained in rotten meat and feces would've been adapative since our species split off on its own, and revulsion in the face of infectious sores, etc., would've been adaptive for the last 10,000 or so years since infectious disease became widespread among humans (before then, population density, sedentism, and exposure to other animals were not sufficiently high to sustain outbreaks of smallpox, influenza, and so on).
But what happens when we face novel threats of infection? I think that, in 100 years or so, researchers will look back and ask, "What the hell were those people thinking, keeping their toothbrush in the same room as the toilet?" Worse, most people never or only rarely clean their toothbrushes -- and they should really just replace them -- despite the fact that most packages suggest that toothbrushes be replaced every two months. And that's not even to speak of those who don't use anti-bacterial mouthwash after brushing! I doubt that if the instrument were an eating utensil, these practices would be tolerated, even though eating utensils spend less time inside your mouth touching every square millimeter therein. The problem, of course, is a gene-environment mismatch -- usage of toothbrushes has been commonplace for, what, four or five generations? -- such that we don't have programmed visceral reactions to scraping the inside of our mouths with a tool that we never or rarely clean and that's been stored in a not-so-clean place, forgoing any antiseptics afterwards.
I wonder how much money we could save from pushing better oral hygiene (even just rinsing with mouthwash), given how much we surely spend on fixing chronic health problems arising from oral infections.