Viruses in 20: Obligate host — Section 996: Obligate host

Packets of DNA or RNA that are not cells — they replicate only inside living hosts. 20–300 nm for most; some giant viruses exceed 400 nm On the order of 10^31 particles in estimates None independent — host ribosomes required Do not target virions; they target bacteria Viruses are not cells. They are DNA or RNA genomes in protein coats — sometimes wrapped in a stolen membrane — that can make more of themselves only inside a living host.

That host rule explains the whole public story: vaccines train recognition, antibiotics miss the target, and most viral lineages on Earth never notice a human being. This atlas treats viruses as a branch of the microbial world, not a synonym for 'the worst disease'. Later topics cover phages, vaccines and everyday encounters.

Viruses: Read inside the Tree of microbes cluster before cross-atlas jumps. Primary desk: public-health desks and lab teaching frames. Rather than relying on simplified league tables or slogans, keeping the primary Viruses section open beside benchmark 20 (“Obligate host”) gives operators and readers a verifiable decision trail.