To take an example close to home: there are over 500 species of bacteria now known -- friendly bacteria -- living symbiotically in your mouth and throat probably necessary to your health for holding off pathogenic bacteria.
举一个身边的例子：已知有500多种的细菌—— 友好的细菌——正生活在你的口腔里 也许它们正在为你抵抗来自致病细菌的进攻