Bacteria are life forms, which, like all other life forms, struggle for the best living conditions for themselves. Therefore they will try to avoid getting attacked by the human immune system, and therefore they have developed various ways to protect themselves from the human immune system. When safe from the immune system, they can focus on breeding and multiplying, and if they become numerous enough, the human body will experience their presence as an infection. Some bacteria are relatively harmless, while others are fatal...
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