Topic overview

Start with the big picture

The microbial world includes bacteria, viruses, fungi, protozoa, helminths, prions, archaea, and microscopic algae. A central distinction is cellular organization: prokaryotes lack a membrane-bound nucleus and organelles, while eukaryotes possess them. Microbes occur in ordinary environments and in extreme niches, including hot, salty, or cold settings. In humans, normal microbiota inhabit the skin, gut, and mucosae and can support digestion, vitamin synthesis, immune priming, and resistance to colonization by pathogens. Some organisms become opportunistic pathogens when barriers or immunity are compromised. The broader lesson also considers transmission, microbial structure and identification, virulence, biofilms, and the characteristic features of major microbial groups.