Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of the story. The models are simplified to show the idea.
Tap the buttons and see what happens.
Drag the zoom. At what size do you finally see a virus?
Zoom from 20 mm down to ~300 nm. Where do bacteria and virions appear?
Slide the zoom. Find the tiny virus!
Zoom levels use real sizes: most viruses are 20–250 nm across.
Tap each part to take it off. What is left without the genes?
Toggle envelope, capsid and genome. Which parts does every virus have?
Tap the parts to take them off.
Pick something and run the checklist. Which boxes does a virus miss?
Compare a cat, a bacterium, a crystal and a virus against the textbook criteria.
Tap a thing. Is it alive?
A simplified checklist; "?" marks points scientists debate.
Drop a phage onto the bacterium, then scrub through each stage.
Scrub attachment, injection, synthesis, assembly and lysis.
Tap Drop and watch!
T4 phage in E. coli: about 30 minutes and 100–200 new phages.
Drag the red line. Does a virus count as alive under your rule?
Move the boundary between definitions. Which definition excludes viruses but keeps parasitic bacteria?
Our simplified summaries of different definitions of life.
Tap each idea. Which one sounds most likely to you?
Compare the escape, reduction and virus-first hypotheses.
Tap an idea.
Related: what causes sneezing · how vaccines work · why DNA is important
When you have a cold, you sniff and sneeze. A cold comes from tiny things called viruses. They are much too small to see, even smaller than the germs called bacteria.
A virus is very simple. Inside are its instructions. They say how to make more viruses. Around them is a coat, like a tiny box. That's all!
Living things eat, grow and make more of themselves. A cat does all of that. A virus doesn't eat. It doesn't grow. And it can't make copies on its own. So most scientists say a virus is not alive.
So how does a virus make copies? It borrows a cell! It fits onto the cell like a key in a lock. It puts its instructions inside. Then the cell makes lots of new viruses.
Scientists are still finding out. There are lots and lots of viruses in the sea!
Want more? Find out why we sneeze, how vaccines work and why DNA is important.
Note: This page explains how viruses work; it isn't medical advice.
Viruses are not considered living because they can't do anything on their own. A virus is just a set of genes wrapped in a protein coat. It has no cells, no metabolism of its own (it can't make energy by itself), it doesn't grow, and it has no machinery to build proteins.13 Outside a living cell, a virus is an inert particle that simply drifts.4
Viruses do make copies of themselves, but only by getting inside a living cell and taking over its machinery. The cell does all the work.2 So by the strict checklist biologists use for life, most scientists say viruses are not alive. Some scientists disagree, and call viruses a grey area at the very edge of life.56
A virus is a tiny package with just two or three parts:25
That's the basic plan (some viruses also carry a few enzymes). There are no cell parts inside, no ribosomes to build proteins, and no way to make energy.3 One microbiologist jokingly calls a virus "gift-wrapped" genetic material.5
Viruses are also incredibly small. Most are about 20 to 250 nanometres across (a nanometre is a millionth of a millimetre).2 A typical bacterium is 0.1 to 5 micrometres, so it's many times bigger, and your own cells are bigger still, at 10 to 100 micrometres.9 Scientists only got their first good look at viruses with the electron microscope, in the late 1930s.2
Biology textbooks list the features all living things share: order, responding to the environment, reproduction, adaptation, growth and development, regulation (keeping conditions steady inside), energy processing, and evolution.1 Here's how a virus compares with three other things:
| Feature | Cat | Bacterium | Crystal | Virus |
|---|---|---|---|---|
| Made of cells | ✓ | ✓ (one) | ✗ | ✗ |
| Uses energy (metabolism) | ✓ | ✓ | ✗ | ✗ |
| Grows | ✓ | ✓ | sort of (adds atoms) | ✗ |
| Responds to its surroundings | ✓ | ✓ | ✗ | debated |
| Has genes | ✓ | ✓ | ✗ | ✓ |
| Makes copies of itself | ✓ | ✓ | ✗ | only inside a cell |
| Evolves | ✓ | ✓ | ✗ | ✓ |
The cat and the bacterium tick every box on their own. A crystal can "grow" by adding atoms, but it has no genes and uses no energy. The virus is the odd one out: it has genes and it evolves (which is why flu vaccines are updated every year), yet it has no cells, no metabolism, and can't copy itself without a host.34 Whether a virus really "responds" is debated: it binds to cells, but only because of its shape and chemistry.4 The ✓ and ✗ marks in the table and in the player's test are our simple summary of these sources, not an official scoring system.
It borrows a cell. A good example is a bacteriophage ("phage" for short), a virus that infects bacteria. The T4 phage, which infects the gut bacterium E. coli, looks like a tiny lunar lander: a head full of DNA on top of a tail with spindly fibres.210 Most virus infections follow the same basic steps:8
Notice what's missing: the virus never eats, never makes energy and never builds anything itself. That's the heart of why viruses are not considered living. The same thing happens when you catch a cold: a few virus particles get into the cells lining your nose, and a few days later huge numbers of new viruses have been made.3 More than 200 different viruses can cause colds; rhinoviruses are the most common.11 Bacteria are different: they are living cells with their own protein-making machinery, and most reproduce all by themselves by splitting in two.
Not every scientist agrees that viruses are simply not alive, and the arguments are worth hearing:56
So viruses sit in a grey area. Scientific American describes them as "verging on life".6 Where you draw the line depends on how you define life: if life means "genes that can evolve", viruses count; if it means "can make its own proteins and energy", they don't. The "line of life" toy in the player lets you try different lines. Its rules are our simplified summaries of these positions, not official definitions.
You might wonder why something that isn't alive exists at all. The answer is that anything that gets itself copied more often than it's destroyed will stick around, and viruses are extremely good at getting cells to copy them. They're also hugely successful: there are thought to be around 1031 virus particles in the oceans (a one with 31 zeros), far more than all other living things on the planet put together.5
Viruses aren't just freeloaders, either. They move genes between living things. Some phages carry genes from one bacterium to another.12 And about 8% of the human genome is made of the remains of ancient viruses that got into the DNA of our distant ancestors and were passed down ever since.7 (DNA is the instruction book of life; see why DNA is important.)
Nobody knows for sure, and viruses leave no fossils. Scientists have three main ideas:3
Different kinds of virus may have started in different ways, and the answer may turn out to be a mix of all three.3
Because viruses aren't living cells. Antibiotics work by attacking parts of bacteria, such as their cell walls or their protein-making machinery. Viruses don't have those parts, so antibiotics can't touch them. That's why antibiotics don't help with colds, flu or most sore throats.13 (See how antibiotics work.)
What helps against viruses? Washing your hands with soap helps stop them spreading.16 Vaccines train your immune system to recognise a virus before you meet it,17 and antiviral medicines can block steps of a particular virus's copying cycle.14 Learn more in how vaccines work, or see how one especially slow virus travels through the body in how rabies works. And a nice twist: because phages kill only bacteria, doctors in some countries use them to treat bacterial infections, and in the US phage sprays are approved to kill Listeria germs on meat.14
Because they fail most of the checklist biologists use for life. A virus is not made of cells, has no metabolism of its own (it can't make energy by itself), doesn't grow, and has no ribosomes to build proteins. It can only make copies of itself by getting inside a living cell and using that cell's machinery. Outside a cell, a virus is an inert particle.
Something doesn't have to be alive to be copied. Any package of genes that gets itself copied more often than it's destroyed will stick around, and viruses are very good at getting cells to copy them. Scientists think viruses are extremely ancient. They may have come from genes that escaped from cells, from cells that lost parts, or they may be as old as cells themselves. Nobody knows for sure.
There are three main ideas. The escape idea says viruses began as bits of genetic material that gained the ability to move between cells. The reduction idea says they were once free-living cells that became parasites and lost genes. The virus-first idea says viruses came before, or alongside, the first cells. Different kinds of virus may have started in different ways.
Viruses have no cells, no cell membrane of their own, no ribosomes and no metabolism, so they can't make energy or proteins. They don't grow or develop, and they can't reproduce on their own. Outside a host cell they can't grow, make energy or make copies.
Viruses have genes made of DNA or RNA, they reproduce (with a host cell's help), and they evolve over time by natural selection. That's why flu vaccines are updated every year. These traits are why some scientists say viruses sit at the edge of life.
Because they tick some boxes of life and not others. Outside a cell they behave like chemistry; inside a cell they take over and multiply. Giant viruses such as mimivirus, with more than a million DNA letters and some protein-making genes, blur the line even more. Whether viruses count as alive depends on exactly how you define life.
No. Antibiotics work on bacteria, which are living cells with their own machinery to attack. Viruses don't have that machinery, so antibiotics don't work on colds, flu or other viral infections. Most viruses never make people sick. Washing your hands with soap and water helps stop the ones that do from spreading, vaccines help prevent some viral infections, and antiviral medicines can treat some of them. This page explains how viruses work; it isn't medical advice.