Microbiota of the oral cavity number in the millions and include archaea, bacteria, fungi, protists, and viruses. These organisms exist together and most in a mutualistic relationship with the host, where both the microbes and the host benefit from the relationship. While the majority of oral microbes are beneficial, preventing harmful microbes from colonizing the mouth, some have been known to become pathogenic in response to environmental changes. Bacteria are the most numerous of the oral microbes and include Streptococcus, Actinomyces, Lactobacterium, Staphylococcus, and Propionibacterium.