Discovery Trail — how the invisible was found
Until the late 1930s and the advent of the electron microscope, no one had ever seen a virus. Yet prevention was already being practiced: by the 17th century (perhaps earlier), inoculation (variolation) was used against smallpox in various parts of the world — and by the late 18th century the Englishman Edward Jenner was inoculating patients with cowpox to prevent smallpox, a technique he named vaccination. This station retraces the laboratory trail that revealed the viral world. Run the famous filter experiment, then walk the timeline.
Viral Structures — the anatomy of a virion
Viruses are acellular — not made of cells, and not included in the tree of life; their evolutionary origin is still a matter of speculation. To survive and reproduce they must infect a cellular host: they are obligate intracellular parasites. Click the glowing points on the virion to explore its parts — then check the size bar: a virion runs from 20 nm up to 900 nm.
Host Range — who can a virus infect?
Viruses can infect every type of host cell — plants, animals, fungi, protists, bacteria and archaea. But most viruses only infect the cells of one or a few species: that is their host range. A wide host range is not common, and even within a host, viruses typically attack only specific cell types. Viruses that infect bacteria are bacteriophages (“phages”).
Transmission — how viruses travel
Viruses move between hosts by direct contact, indirect contact with fomites, or through a vector — an animal that transmits a pathogen from one host to another. Arthropods such as mosquitoes, ticks and flies are typical vectors.
Taxonomy & Genomes — naming the unnameable
Viruses are not classified in the three domains of life — but their numbers are great enough to demand classification. Since 1971, the International Union of Microbiological Societies Virology Division has tasked the International Committee on Taxonomy of Viruses (ICTV) with developing and maintaining a universal virus taxonomy. Because viruses mutate so quickly, the binomial system is hard to apply — so the ICTV classifies them into families and genera based on genetics, chemistry, morphology and mechanism of multiplication. To date: 7 orders, 96 families, 350 genera.
Lytic Cycle — the virulent phage simulator
During the lytic cycle of a virulent phage, the bacteriophage takes over the cell, reproduces new phages, and destroys the cell. The T-even phages are the classic example. Step through the five stages — and watch the host pay the price.
Lysogenic Cycle — the quiet option
In the lysogenic cycle, the phage genome enters through the same attachment and penetration — but instead of killing the host, it integrates into the bacterial chromosome and becomes part of it. The classic example is the lambda phage. Step through the stages — and note how easily the quiet option can turn loud.
Chapter Quiz — Acellular Pathogens
Twenty-three questions covering the whole chapter: the discovery of viruses, viral structures and taxonomy, hosts and transmission, and the lytic and lysogenic cycles.