A computer virus is a kind of software that gets in the way of a computer's normal work. It can close programmes on its own, keep showing the same message again and again, or shut the computer down without being asked to.
Why it is called a virus
The name is borrowed from biology, and it is not just a colourful comparison. A biological virus cannot live on its own. It gets inside a living cell, uses that cell to make copies of itself, and those copies go on to reach other cells. A computer virus behaves the same way. It cannot run on its own, so it hides inside a file or a programme, and when that file is opened it makes copies of itself which travel to other files, other pen drives, other computers.
Hold on to those two ideas, because they explain nearly everything else about viruses. It needs a host, so it has to be carried by something you open. It copies itself, so one infected pen drive passed around a classroom can leave every computer in the room infected by the end of the day.
A virus is a programme somebody wrote on purpose
It helps to be clear about what a virus actually is, because the word makes it sound like something that grows by itself. It does not. A virus is ordinary software. A person sat down and wrote it, the same way a person wrote the calculator or the drawing programme on your computer. The only difference is what the instructions tell the computer to do.
This raises an obvious question. If the instructions are harmful, why does the computer follow them? The answer is the single most important idea on this page: a computer does not judge. It cannot tell the difference between an instruction you wanted and an instruction you did not want. It only knows how to obey. Delete this file is a perfectly ordinary instruction, and the computer carries it out exactly the same way whether it came from you or from a virus. That is why protection has to happen before the instruction runs, not after.
How we notice a virus
If a computer keeps having trouble doing simple tasks, or switches itself on or off without being told to, a virus could be the reason. Other signs include files that refuse to open, folders that will not close or delete, unfamiliar folders appearing on a pen drive or memory card by themselves, and messages popping up while nothing was asked for. When several of these things happen together, it is worth checking whether the computer has a virus.
This is where people go wrong most often. A slow computer is usually not infected. It is far more likely to be old, to have too many programmes open at once, or to have a storage disk that is almost completely full. Blaming a virus straight away is exactly the mistake Unit 1 warned about: jumping to a conclusion instead of testing it. Close everything, check how much free space is left, restart, and see whether the problem is still there. Only when the ordinary explanations have been ruled out is a virus the sensible answer.
The signs that point strongly to a virus are the ones a tired old machine cannot explain. A folder you never made, appearing on your pen drive by itself. Files renamed without anyone touching them. The same message returning every few minutes no matter how often you close it. Those are not slowness. Those are something acting on its own, and something acting on its own is what a virus is.
How a virus spreads
The internet is the main way a virus enters a computer. Opening a website whose files carry a virus can let it straight into the host computer. Devices such as a pen drive, a memory card, a CD, or a hard disk can also carry a virus from one computer to another. If any of these already have a virus on them, simply plugging them into a different computer can spread the virus there too. Some viruses even install themselves automatically while another programme is being installed, without the user realising it.
Notice what every one of those routes has in common. Something came in from outside, and something was opened. A file downloaded, a pen drive plugged in, a programme installed. A virus cannot walk across a room. It needs a door, and the door is always something arriving from somewhere else. That is why the shared pen drive in a school computer lab spreads more infections than anything else in the building.
Protecting a computer from viruses
Installing a suitable antivirus programme is the main way to protect a computer. Once installed, its virus database needs to be kept updated, since new viruses appear all the time. Running a full scan or a quick scan lets the antivirus check the computer and report anything suspicious it finds, so it can be removed before it causes real damage.
Why updating matters so much
An antivirus programme mostly works by recognition. It keeps a long list of known viruses, rather like a book of wanted posters, and checks every file it meets against that list. If a file matches something in the book, it is stopped.
Now the problem becomes obvious. New viruses are written every single day, and a book of posters printed last year has no picture of this year's face. An antivirus that has not been updated is not broken, it is simply looking for the wrong things. It will scan happily, report that everything is clean, and be completely wrong. This is why updating is not an optional extra, and it is why a free antivirus that is kept up to date protects you better than an expensive one that is two years out of date.
It also means an antivirus can never be a complete answer. It catches what it already knows about, which leaves a gap for anything brand new. The person using the computer is the part that fills that gap, by not opening the attachment from a stranger and by scanning the pen drive before trusting it. The programme is the second line of defence. You are the first.
| Habit | What it stops |
|---|---|
| Keep the antivirus database updated | Viruses written after your last update, which the old list cannot recognise |
| Scan any pen drive or memory card before opening it | The most common route of all in a school, one shared drive passed between many machines |
| Do not open attachments from people you do not know | Brand new viruses that no antivirus list has caught up with yet |
| Install programmes only from places you trust | Viruses that install themselves quietly alongside something you did want |
| Keep a copy of important files somewhere separate | Nothing, but it means a virus that does get through cannot cost you your work |
Keep antivirus updated
New viruses appear constantly, so an antivirus programme that is not updated may fail to recognise them.
Scan unfamiliar devices
Before opening files from a pen drive or memory card that came from another computer, scanning it first can catch a virus before it spreads.
Back up what matters
A second copy kept somewhere else does not prevent a virus, but it decides whether an infection is an annoyance or a disaster.
You are the first defence
An antivirus only catches what it has seen before. The choice not to open a suspicious file is the part no programme can make for you.
Ignoring small, strange behaviour on a computer, thinking it will fix itself, is a common mistake. A file that suddenly will not open, or a folder that appears from nowhere, is worth checking right away rather than waiting. A virus does not get tired and stop. Every hour it is left alone is another hour it spends making copies.
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