Envelope
A fatty outer layer borrowed from a host cell. Soap can break apart this layer, which is why handwashing helps against many enveloped viruses.
Real example: Influenza viruses and SARS‑CoV‑2 have envelopes. Norovirus does not.
Biology, computing, and real-world examples
What is a virus?
A virus is a tiny package of genetic instructions. It can enter a living cell and use that cell’s machinery to make more copies of itself. Unlike cells, viruses cannot reproduce on their own.
Not a cellNo metabolism, ribosomes, or independent growth
Needs a hostViruses can infect cells from animals, plants, fungi, bacteria, and more
Ancient and diverseViruses infect every known form of life
01 · The basic idea
Open up a virus
Tap a labeled part of this simplified virus. Different viruses have different shapes, but many use the same basic ingredients.
A generic enveloped virus, enlarged enormously. It is not drawn to scale.
Genetic code in a coat
Usually DNA or RNA inside a protein shell. Some have an envelope. They have no cell parts for making proteins or energy.
A complete living cell
Can grow and reproduce on its own. Antibiotics can target some bacteria, but do not kill viruses.
A practice lesson for immunity
It gives the immune system safe information about a germ, helping it recognize a real threat faster later.
Scale tool
How small are viruses?
A typical virus is far too small to see with a regular light microscope.
Human cell
10–100 μm
10–100 μm
Bacterium
1–10 μm
1–10 μm
Virus
20–300 nm
20–300 nm
Protein
~5 nm
~5 nm
1 micrometre (μm) = 1,000 nanometres (nm).
02 · A host is required
The infection cycle
Viruses are specialists. A virus that infects a human lung cell may be unable to infect a plant, a bacterium, or even a different kind of human cell.
Cell DNA
Attach: finding the right lock
Proteins on the virus surface fit particular molecules on the outside of a host cell, a bit like a key fitting a lock. That match helps decide which cells it can infect.
Example: HIV mainly targets immune cells with a receptor called CD4.
03 · Interactive model
Watch a simple outbreak
Change the conditions, then run the model. This is a teaching simulation, not a prediction of a real disease outbreak.
Can catch it
Infected
Recovered
Day0
Currently infected1
Ever infected1
The dots are imaginary people. Real outbreaks also depend on immunity, ventilation, behavior, testing, treatment, chance, and many other factors.
Live chart
Infections over time
04 · Real viruses, real hosts
Virus field guide
Viruses are not only human diseases. They shape oceans, ecosystems, evolution, food production, and scientific research.
Influenza A
A family of RNA viruses behind seasonal flu. Some strains circulate in birds or pigs too, which is why scientists monitor animal flu viruses.
SARS‑CoV‑2
A coronavirus first identified in 2019. It mainly spreads through air people share, especially in close or poorly ventilated indoor settings.
Human papillomavirus
HPV is a large group of viruses. Some types can cause cancers years after infection; vaccines prevent several high-risk types.
Rabies virus
A virus that infects mammals and attacks the nervous system. Vaccination after a possible exposure can prevent disease when given quickly.
Tobacco mosaic virus
One of the first viruses ever discovered. It causes mottled patterns on leaves and helped show that infectious agents could be smaller than bacteria.
Bacteriophage
A “phage” is a virus that infects bacteria. Phages are extremely common and may one day help fight some antibiotic-resistant infections.
Avian influenza
Flu viruses adapted to birds. Most do not spread easily between people, but monitoring them helps detect changes early.
Ocean phages
Viruses infecting ocean microbes influence nutrient cycles and may affect how carbon moves through marine ecosystems.
05 · Why computers use the same word
Biological viruses vs. computer viruses
The name is a metaphor. Both spread by making copies with help from a host, but one is biology and the other is software.
Question
Biological virus
Computer virus
What is the host?
A living cell
A computer, program, or network
What gets copied?
DNA or RNA instructions
Computer code
How can it spread?
Air, contact, insects, fluids, food, and more
Files, links, software flaws, removable drives, and networks
How do we reduce risk?
Vaccines, ventilation, hygiene, testing, treatment
Updates, backups, careful downloads, security software
Try the analogy sorter
Choose whether each idea fits biology, computing, both, or neither.
06 · Knowledge check
Can you spot the key ideas?
Your best score and lab note save when storage is available. Review each explanation before moving on.
Question 1 of 5
Your lab note
Explain it in your own words
Try: “A virus is different from a bacterium because…”