How does a computer get infected with virus

Though they're not a living thing like you and me, computers can get "sick" from viruses, too.

Table of Contents Show

Frequently Asked Questions
For More Information
Frequently Asked Questions
How to get rid of a computer virus
Removing a computer virus from a PC
Removing a computer virus from a Mac
What is a computer virus?
How to tell if your computer has a virus
Can your computer become infected with a virus via email?
How to protect your computer from viruses

A computer virus is a software program designed to replicate itself and spread to other machines. In most cases, the program is "malicious," meaning its purpose is to cause the computer malfunction in some way.

In general usage, the term "computer virus" includes all forms of "malware," or malicious software.

Instead of sniffles and a fever, some common symptoms of a computer viral infection are slow performance, data loss and system crashes, all of which can make people using the machine feel ill as well.

Yet many technological diseases were designed to remain hidden on a computer and not alert its user, so malware-infected machines may spread silently.

Since Elk Cloner, the first computer virus, left its lab in 1981, millions more have been created by human programmers who have decided to sabotage others' computers.

"There is always something new," said Peter Szor, an independent researcher and former engineer at Symantec Corporation, a maker of antivirus software and author of The Art of Computer Virus Research and Defense (Addison-Wesley, 2005). Symantec and other virus labs often see more than 30,000 unique malware programs on a single day, Szor added.

The sinister programs often work by associating themselves with a legitimate program that when activated also "executes," or runs, the virus' code.

Viruses can enter your computer in any number of ways, such as via an email attachment, during file downloads from the Internet or even upon a visit to a contaminated Web site.

Digital germs

Sort of like how bacteria and fungi cause illnesses in people, computers can also get diseases from other infectious agents besides viruses, including computer worms, Trojan horses and spyware.

Unlike viruses, worms do not have to attach themselves to a program in your computer, and may not damage files on an infected computer. Instead, worms more often slow down computer networks by eating up bandwidth, or your computer's ability to prcess data, as the malware replicates and spreads.

Trojan horses, on the other hand, do not self-replicate. Instead, these programs act as the sneaky means for a hacker to gain access to someone's computer to send out spam emails or steal passwords.

Spyware programs monitor a computer user's activity, such as Web sites they visit, without the user knowing it, and may cause unsolicited advertisements to pop up ("adware") or may steal sensitive information such as credit card numbers.

A whole range of antivirus software is available to prevent and eradicate these malware infections.

"Antivirus programs are evolving to keep up with new threats," said Szor. "My recommendation to users today is to buy the most recent version of the products."

This information gives an introduction to viruses and ways to avoid them.

Frequently Asked Questions

For More Information

US-CERT offers many resources to help you create a more secure home computing environment. These documents may be of particular interest if you have concerns about viruses and Trojan horses:

Frequently Asked Questions

What is a virus?
A computer virus is a program that spreads by first infecting files or the system areas of a computer or network router's hard drive and then making copies of itself. Some viruses are harmless, others may damage data files, and some may destroy files. Viruses used to be spread when people shared floppy disks and other portable media, now viruses are primarily spread through email messages.

Unlike worms, viruses often require some sort of user action (e.g., opening an email attachment or visiting a malicious web page) to spread.

What do viruses do?
A virus is simply a computer program--it can do anything that any other program you run on your computer can do. Some viruses are designed to deliberately damage files, and others may just spread to other computers.

What is a worm?
A worm is a type of virus that can spread without human interaction. Worms often spread from computer to computer and take up valuable memory and network bandwidth, which can cause a computer to stop responding. Worms can also allow attackers to gain access to your computer remotely.

What is a Trojan horse?
A Trojan horse is a computer program that is hiding a virus or other potentially damaging program. A Trojan horse can be a program that purports to do one action when, in fact, it is performing a malicious action on your computer. Trojan horses can be included in software that you download for free or as attachments in email messages.

Can I get a virus by reading my email messages?
Most viruses, Trojan horses, and worms are activated when you open an attachment or click a link contained in an email message. If your email client allows scripting, then it is possible to get a virus by simply opening a message. It's best to limit what HTML is available in your email messages. The safest way to view email messages is in plain text.

How can I avoid a virus infection from email?
Most users get viruses from opening and running unknown email attachments. Never open anything that is attached to an email message unless you know the contents of the file. If you receive an attachment from a familiar email address, but were not expecting anything, you should contact the sender before opening the attachment. If you receive a message with an attachment and you do not recognize the sender, you should delete the message.

Selecting the option to view your email messages in plain text, not HTML, will also help you to avoid a virus.

What are some tips to avoid viruses and lessen their impact?

Install anti-virus software from a reputable vendor. Update it and use it regularly.
In addition to scanning for viruses on a regular basis, install an "on access" scanner (included in most anti-virus software packages) and configure it to start each time you start up your computer. This will protect your system by checking for viruses each time you run an executable file.
Use a virus scan before you open any new programs or files that may contain executable code. This includes packaged software that you buy from the store as well as any program you might download from the Internet.
If you are a member of an online community or chat room, be very careful about accepting files or clicking links that you find or that people send you within the community.
Make sure you back up your data (documents, bookmark files, important email messages, etc.) on disc so that in the event of a virus infection, you do not lose valuable work.

Are you concerned that your computer may have a virus? If your computer is infected, learning how to get rid of a computer virus is vital.

This article teaches you all there is to know about how computer viruses work and computer virus removal.

Read on as we discuss:

How to get rid of a computer virus.
What a computer virus is.
How to tell if your computer has a virus.
Whether your computer can become infected with a virus via email.
How to protect your computer from viruses.

How to get rid of a computer virus

In this section, we explore how to get rid of a computer virus from a PC and from a Mac.

Removing a computer virus from a PC

Computer viruses are almost always invisible. Without anti-virus protection, you may not know you have one. This is why it is vital to install anti-virus protection on all your devices.

If your PC has a virus, following these ten simple steps will help you to get rid of it:

Step 1: Download and install a virus scanner

Download a virus scanner or complete internet security solution. We recommend Kaspersky Internet Security. The video below will guide you through the installation process:

Step 2: Disconnect from internet

When you are removing a virus from your PC, it is a good idea to disconnect from the internet to prevent further damage: some computer viruses use the internet connection to spread.

Step 3: Reboot your computer into safe mode

To protect your computer while you remove the virus, reboot it in ‘Safe Mode’. Are you unsure of how to do this?

Here is a simple guide:

Turn your computer off and on again
When the screen lights, press F8 to bring up the ‘Advanced boot options’ menu
Click ‘Safe Mode with Networking’
Remain disconnected from the internet

Step 4: Delete any temporary files

Next, you need to delete any temporary files using ‘Disk Clean Up’.

Here’s how to do this:

Click the Windows logo on the right bottom
Type “Temporary Files”
Choose “Free up disk space by deleting unnecessary files”
Find and select “Temporary Internet Files” in the ‘Files to delete’ Disk Cleanup list and click OK
Confirm “Delete Files” selection

Some viruses are programmed to initiate when your computer boots up. Deleting temporary files may delete the virus. However, it is not safe to rely on this. To ensure you rid your computer of viruses, it is wise to complete the following steps.

Step 5: Run a virus scan

Now it is time to run a virus scan using your chosen anti-virus or internet security software. If you are using Kaspersky Internet Security, select and run ‘Scan’.

Step 6: Delete or quarantine the virus

If a virus is found, it may affect multiple files. Select ‘Delete’ or ‘Quarantine’ to remove the file(s) and get rid of the virus. Rescan your computer to check there’s no further threats. If threats are found, quarantine or delete the files.

Step 7: Reboot your computer

Now that the virus is removed, you can reboot your computer. Simply turn it on as you would normally. It no longer needs to be in ‘Safe Mode’.

Step 8: Change all your passwords

To protect your computer from further attack, change all your passwords in case they were compromised. This is only strictly necessary if you have reason to believe your passwords have been captured by malware, but it is better to be safe than sorry.

You can always check the virus’s functionality on your anti-virus vendor’s website or with their technical support team if unsure.

Step 9: Update your software, browser and operating system

Updating your software, browser and operating system will reduce the risk of flaws in old code being exploited by criminals to install malware on your computer.

Removing a computer virus from a Mac

If you use a Mac, you may be under the impression that your computer cannot get a virus. Unfortunately, this is a misconception. There are fewer viruses that target Macs compared with the many that target PCs, but Mac viruses do exist.

Some Mac viruses are designed to trick users into thinking they are anti-virus products. If you accidentally download one of these, your computer may be infected. Three examples of Mac viruses of this type are ‘MacDefender’, ‘MacProtector’, and ‘MacSecurity’.

If you think your Mac has a virus, here are six steps to follow to remove it:

Quit the application or software that seems to be affected.
Go to ‘Activity monitor’ and search for known Mac viruses such as ‘MacDefender’, ‘MacProtector’, or ‘MacSecurity’.
If you find one of these viruses, click ‘Quit process’ before quitting ‘Activity monitor’.
Next, go to your ‘Applications’ folder and drag the file into your ‘Trash’.
Remember to empty the ‘Trash’ folder afterwards to permanently delete the virus.
Now make sure your software and apps are up to date to benefit from the latest security patches.

To ensure nothing is missed and to keep your Mac protected, consider installing a running an anti-virus solution if you do not already have one. We recommend comprehensive internet security solution like Kaspersky Total Security.

What is a computer virus?

A computer virus is a type of malware (malicious software) designed to make self-replicate, i.e. to make copies of itself on any drive connected to your computer.

Computer viruses are so-called because, like real viruses, they can self-replicate. Once your computer is infected with a virus, this is how it spreads. When a computer virus infects your computer, it may slow it down and stops it working properly.

There are three main ways that your computer may have become infected with a computer virus.

The first way your computer could become infected from removable media, like a USB stick. If you insert a USB stick or disk into your computer from an unknown source, it may contain a virus.

Sometimes hackers leave infected USB sticks or disks in people’s workplaces, or public places like cafes to spread computer viruses. People sharing USBs may also transfer files from an infected computer to one that isn’t infected.

Another way your computer become infected with a virus is through a download from the internet.

If you are downloading software or apps to your computer, ensure you do so from a trusted source. For example, the Google Play Store or Apple’s App Store. Avoid downloading anything via a pop-up or a website you do not know.

The third way your computer could become infected with a virus is if you open an attachment, or click on a link, in a spam email.

Whenever you receive mail from a sender you do not know or trust, avoid opening it. If you do open it, do not open any attachments or click on any links.

How to tell if your computer has a virus

There are numerous signs to look out for that indicate your computer may have a virus.

Firstly, is your computer slowing down? If everything is taking longer than usual, your computer may have become infected.

Secondly, look out for apps or programs that you do not recognize. If you see an app or a program appear on your computer that you do not remember downloading, exercise caution.

It is a good idea to uninstall any software you do not recognize and then to run a virus scan using anti-virus or internet security software to check for threats. Pop-ups that appear when your browser is closed are a tell tail sign of a virus. If you see these, take immediate action to remove the virus, by following the steps outlined above.

Another sign that your computer may have a virus is if apps or programs on your computer start behaving strangely. If they start crashing for no apparent reason, your computer may have a virus.

Finally, a virus may cause your computer to start overheating. If this happens, investigate whether you have a virus using anti-virus or internet security software.

Can your computer become infected with a virus via email?

Your computer can become infected with a virus via email, but only if you open attachments within a spam email or click on the links contained in them.

Simply receiving a spam email will not infect your computer. Just mark these as spam or junk and ensure they are deleted. Most email providers will automate this (Gmail for example) but if any slip through the net, just mark them as spam yourself and don’t open them.

How to protect your computer from viruses

Here are some key ways that you can protect your computer from viruses:

Use anti-virus software or a complete internet security solution, like Kaspersky Total Security. For your Android mobile, consider Kaspersky Internet Security for Android.
Research apps and software by reading user reviews.
Read developer descriptions before you download apps and software.
Only download apps and software from trusted sites.
Check how many downloads apps and software have. The more, the better.
Check what permissions apps and software ask for. Are these reasonable?
Never click on unverified links in spam emails, messages or unfamiliar websites.
Do not open attachments in spam emails.
Keep software, apps and your operating system updated.
Use a secure VPN connection for public Wi-Fi, like Kaspersky Secure Connection.
Never insert unknown USB sticks or disks into your computer.

Why expose yourself to the risk of infection? Protect your computer with Kaspersky Total Security.

Kaspersky Internet Security received two AV-TEST awards for the best performance & protection for an internet security product in 2021. In all tests Kaspersky Internet Security showed outstanding performance and protection against cyberthreats.

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Has your computer got a virus? Learn how to get rid of a computer virus, how to find out if your computer has one and if you can get a computer virus via email.