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Oral hygiene is the practice of keeping one's oral cavity clean and free of disease and other problems (e.g. bad breath) by regular brushing of the teeth (dental hygiene) and adopting good hygiene habits. It is important that oral hygiene be carried out on a regular basis to enable prevention of dental disease and bad breath. The most common types of dental disease are tooth decay (cavities, dental caries) and gum diseases, including gingivitis, and periodontitis.

General guidelines for adults suggest brushing at least twice a day with a fluoridated toothpaste: brushing before going to sleep at night and after breakfast in the morning. Cleaning between the teeth is called interdental cleaning and is as important as tooth brushing. This is because a toothbrush cannot reach between the teeth and therefore only removes about 50% of plaque from the surface of the teeth. There are many tools available for interdental cleaning which include floss, tape and interdental brushes; it is up to each individual to choose which tool they prefer to use.

Sometimes white or straight teeth are associated with oral hygiene. However, a hygienic mouth can have stained teeth or crooked teeth. To improve the appearance of their teeth, people may use tooth whitening treatments and orthodontics.

The importance of the role of the oral microbiome in dental health has been increasingly recognized. Data from human oral microbiology research shows that a commensal microflora can switch to an opportunistic pathogenic flora through complex changes in their environment. These changes are driven by the host rather than the bacteria. Archeological evidence of calcified dental plaque shows marked shifts in the oral microbiome towards a disease-associated microbiome with cariogenic bacteria becoming dominant during the Industrial Revolution. Streptococcus mutans is the most important bacteria in causing caries. Modern oral microbiota are significantly less diverse than historic populations. Caries (cavities), for example, have become a major endemic disease, affecting 60-90% of schoolchildren in industrialized countries. In contrast, dental caries and periodontal diseases were rare in pre-Neolithic and early hominins.