• Even though bacteria and archaea look similar, their DNA shows that archaea are much closer to eukaryotes than to bacteria.
• Bacteria create most of the genetic variety among living things.
• Less than 1% of microbes cause disease in humans.
• Most microbes are either harmless or helpful (and necessary) for human health and for the environment.
• Humans have found ways to use microbial diversity to our advantage (for example, in making medicines, food, and cleaning).
• Microbes live with hosts (like us) through three main types of relationships:
  
• Mutualism (both benefit)
• Commensalism (one benefits, the other is unaffected)
• Parasitism (one benefits by harming the other)
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• Pathogenic (disease-causing) microbes are either:
  
• Opportunistic (cause disease when the host’s defenses are weakened) or
• Exogenous (come from outside the body to cause disease)
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