Gut residents
Help digest fibres, make some vitamins, and crowd out some pathogens.
A healthy person is an ecosystem. Barrier surfaces carry dense communities that train immunity and occupy niches invaders want.
Pathogenesis is ecology gone wrong for us: toxins, invasion, inflammation. Most bacterial cells you meet never cross that line.
Body interfaces
Skin film
Acid and occupancy make it harder for opportunists to bloom.
Wound invasion
Broken barriers let environmental or resident species reach tissue and blood.
Toxin disease
Sometimes the molecule ? tetanus toxin, cholera toxin ? does the damage more than the cell count.
Immune training
Early-life exposure patterns shape tolerance; the story is still being rewritten.
Drug collateral
Broad-spectrum antibiotics save lives and also prune allies; stewardship is the compromise.
Handwashing, vaccines and antibiotics are tools with different jobs ? none of them mean 'delete all bacteria'.
Keep exploring
More in Tree of microbes
Viruses
Packets of DNA or RNA that are not cells — they replicate only inside living hosts.
Familiarity 96Fungi
A eukaryotic kingdom of yeasts, moulds and mushrooms — chitin walls, external digestion, partners of plants.
Familiarity 80Archaea
A prokaryotic domain that is not bacteria — methanogens, extremophiles and no known human pathogens.
Familiarity 35Bacteriophages
Viruses that infect bacteria — Earth's most abundant biological entities, and a concept-level idea in medicine.
Familiarity 42Protozoa
Motile unicellular eukaryotes — a teaching grade that includes pond ciliates and a minority of serious parasites.
Familiarity 58