A computer virus is a type of malicious software (malware)4, meaning a computer program or script.¹ Viruses attempt to spread from one file to another on your computer and/or from one computer to another, using a variety of methods, without your knowledge and consent.¹

When the virus is executed, it replicates itself by modifying other computer programs and inserting its own code. When this replication succeeds, the affected areas on your device are then said to be "infected" with a computer virus⁴ just like the pathogen virus.

A worm is a specific type of virus. It is a standalone computer program that propagates itself across many computers, usually by creating copies of itself in each computer’s memory Worms often use a computer network to spread themselves, relying on security failures on the target computer to access it.⁶

A Trojan Horse (Trojan) is a software that attempts to infiltrate your computer without your knowledge or consent. The term relates to the Ancient Greek story of the deceptive wooden horse which the Greeks used to enter the independent city of Troy and win the war. A Trojan, similar to its Greek mythological counterpart, often presents itself in one form while it is actually another: An e-mail attachment disguised to appear not suspicious, (e.g. a routine form to be filled in), or by clicking on some fake advertisement on social media.⁷

Spyware is software that runs on your computer and aims to gather information about you or your organization,without your knowledge or consent. it tracks  information, e. g. your habits and sends it to another entity or tailors these patterns for advertisements, it can keep influencing what you see, etc.⁸

Computer viruses can harm your computer by damaging programs, deleting files, or reformatting the hard drive. Other viruses replicate themselves or flood a network with traffic, making it impossible to perform any internet activity. Some viruses  significantly disrupt your system’s performance, sap the computer memory and cause frequent computer crashes.⁵

Where viruses almost always corrupt or modify files on a targeted computer worms  cause at least some harm to the network, even if only by consuming bandwidth. Many worms are designed only to spread, and do not attempt to change the systems they pass through. However worms can cause major disruption by increasing network traffic and other unintended effects.⁶

Unlike computer viruses and worms, Trojans generally do not attempt to inject themselves7 into other files or  propagate themselves. Trojans typically do one of two things: they either destroy or modify data the moment they launch, such as erase a hard drive, or they attempt to ferret out and steal passwords, credit card numbers, and other such confidential information.¹

Common tactics for surreptitious installation include rolling up advertising programs into "free" shareware program downloads. Once the spyware is installed at your device it can download advertisements 24 hours a day and overlay them on websites and programs you are using.¹

Tips to deal with malware

How do you know that you have malware on your computer?

You can look out for the following signs which can indicate that you have malware on your computer^¹:

• Your computer runs slowly.
• Your computer drains its battery quickly.
• Your computer displays unexpected errors or crashes.
• Your computer won't shutdown or restart.
• Your computer serves a lot of popups.
• Your computer takes you to web pages you didn't visit, changes your home page, or creates new icons or toolbars without your permission.
• Your computer has unexplained data loss.

 What can you do to protect your computer from malware?²

• You can install a security software/antivirus protection from a reliable company and a firewall.
• You can get an anti-spyware software, which combats spyware from being installed.
• You can always keep your security software up-to-date.
• You can update your operating system regularly.
• You should NOT buy security software in response to unexpected calls or messages, especially if they say they scanned your computer and found malware. 
• You can increase your browser security settings.
• You can avoid questionable Web sites.
• You should only download software from sites you trust.
• You can carefully evaluate free software and file-sharing applications before downloading them.  
• You should shouldn't open messages and links from unknown senders.
• You can immediately delete messages you suspect to be spam.
• You can use a pop up blocker, and don't click on links and popups.

What can you do if you suspect malware on your computer?

If you suspect malware on your computer, you should stop doing activities that require passwords or other personal information, such as online shopping or banking. You should use a different computer than yours to change your passwords. Then you should update your security software and run a system scan and delete files it flags as malware.

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