Viruses, the smallest things that make us ill
A virus is so small that an ordinary microscope cannot see it. It does not eat, does not breathe, does not grow, and cannot do a single thing by itself. But the moment it gets inside one of our cells, it orders that cell to build thousands of new viruses. This lesson looks at what they are made of, how they work, why antibiotics do nothing to them, and how an outbreak can be stopped.
Zoom all the way down
Drag the slider to shrink things one step at a time, from something you can see with the naked eye down to viruses and molecules. A nanometre is one millionth of a millimetre.
What is inside a virus?
A virus carries only a couple of things: a set of genetic instructions, and a box wrapped around them. It has no machinery of its own at all. Tap each part.
Viruses come in several shapes
A virus's shape is not random. It is the result of identical small proteins fitting together by themselves in the way that uses the fewest genetic instructions. Tap to see each one.
How a virus takes over a cell's factory
A virus does not hurt us by biting or stinging. It simply takes over the factory inside a cell and makes that cell build new viruses instead of doing its own job. Press the button to watch the whole cycle.
Which virus causes what?
Tap a virus to see how it spreads, what symptoms it causes and how to prevent it. Notice that when the route of spread is different, the way to prevent it is different too.
Viruses and bacteria are not the same
Both can make you ill, but they are completely different kinds of thing. This is why a doctor does not give antibiotics for a cold.
| Topic | Virus | Bacteria |
|---|---|---|
| Is it a cell? | Not a cell. Only genetic material and a protein coat | A complete cell, with a cell wall, a membrane and all its machinery |
| Size | About 20 to 300 nanometres | About 1 to 5 micrometres, roughly 100 times bigger |
| Multiplying | Only by taking over another cell | Divides by itself, given food and moisture |
| Antibiotics | Do nothing at all because the drugs attack the cell wall and machinery of bacteria, which a virus does not have | Work well, if the right drug is chosen |
| Example diseases | Colds, flu, covid, chickenpox, dengue, rabies | Strep throat, leptospirosis, tuberculosis, some kinds of food poisoning |
| Any use to us? | Yes. Some viruses eat bacteria and can be used as a treatment, and viruses are used as delivery vehicles for genes in modern medicine | Plenty. Gut bacteria help digest food and make vitamins, and they are in yoghurt and fermented milk |
How the body fights a virus
The body has two layers. The first reacts immediately and treats every intruder the same. The second is slower, but precise, and it remembers.
First line, immediate
Skin, mucus, stomach acid and germ-eating cells go to work the moment something foreign appears. Fever is a weapon too, because many viruses multiply more slowly at higher temperatures. A runny nose and a cough are the body trying to push the germs out, not the disease itself.
Second line, precise and remembered
B white blood cells make antibodies that fit that one virus exactly, while T cells destroy infected cells. Once you recover, the body keeps memory cells, so meeting the same virus again is dealt with much faster.
A vaccine is a practice drill
A vaccine takes a piece of a virus, or the instructions for a cell to make that piece, and shows it to the body in advance, without anyone having to be really ill. The body then prepares antibodies and memory cells ready and waiting. A sore arm or a slight fever afterwards is the immune system doing its drill, not an infection.
Why is a flu jab needed every year?
Viruses copy themselves very fast and make copying mistakes often. Small errors build up until the virus looks different, which is called mutation, and the old antibodies no longer grip well. So the flu vaccine has to be reformulated every year. Viruses that change slowly, like measles, give long protection after a full course.
The outbreak sandbox
Each dot is one person. Grey means not infected, red means ill and able to spread it, green means recovered and immune. Try the three sliders on the right, then press start and see how high the curve of ill people climbs.
The highest number of people ill at the same time is what hospitals fear most, because beds and doctors are limited. Flattening the curve means stretching the outbreak out so the system can cope.
How did this germ get here?
Read the card, then choose how this germ travelled to reach you. A run of right answers earns a bonus.
A right answer scores 10 points, and a longer run means a bigger bonus.
Test yourself
Fancy a game on this lesson?
Every question from this lesson is in the Body 101 arena. Race the clock, duel someone at home, play as a team, beat the bosses, or practise with no timer at all.