What is malware? Everything you need to know about malicious software and viruses, and how to protect your computer

Malware refers to any software that is designed with malicious intent — it generally aims to damage, destroy, or steal data, take control of computers, and aid in criminal activity. There are many kinds of malware, so what any specific example of malware depends on what it was created to do. 

Malware has a long history on the PC, with the first known example dating back to 1971 when a virus called Creeper was created. Historically, Windows computers are considered most at risk from malware — not because Macs and Linux are significantly more secure, but because the overwhelming majority of computers run Windows and so hackers and criminals tend to target those machines. 

The different types of malware

Malware is an umbrella term for malicious software. These are the most common kinds of malware and the ones you are most likely to be infected by:

• Virus: Many people mistakenly refer to all malware as a virus, but in reality, a virus is just one, albeit very common, kind of malware. A virus is malware with the ability to replicate itself. For details, read our article on computer viruses. 
• Ransomware: Ransomware became one of the most potent and common forms of malware in the last decade — it encrypts all the files on a computer and demands a ransom to unlock them. You can read more in our article on how to avoid ransomware. 
• Worm: Similar to a virus, a computer worm replicates itself, but does it automatically, without any need for human interaction. You can learn more by reading our article on protecting yourself from computer worms.
• Spyware: This kind of malware steals sensitive information from your PC and sends it to third parties without your knowledge. Find out more about spyware by reading our article on spyware. 
• Adware: Adware is an interesting case — not all adware technically qualifies as malware, even if it's technically unwanted. There are legitimate uses for adware, which promote advertisements on your computer, though some genuinely are malicious and potentially dangerous. Read more in our article on identifying and combatting adware. 
• Trojan: Trojan software is an example that many kinds of malware can have characteristics of more than one sort of malware at once. Trojan software is designed to appear harmless, often masquerading as legitimate software until it has been installed and activated, at which time the malicious payload goes to work. Virtually any other kind of malware — including viruses, ransomware and spyware — can also be a Trojan. 
• Scareware: Scareware is another example of malware that often accompanies other malware payloads. Ironically, viruses and worms often masquerade as antivirus software and display pop-ups telling you that your computer has been infected with malware. The pop-ups recommend downloading a specific program to inoculate your PC. This is additional malware that will further damage your PC. 

How to spot signs you have malware on your computer

Malware takes many forms, but the signs and symptoms of malware are often the same. Here is what you should look for:

Your computer's performance changes suddenly

If you notice your computer suddenly starts running much more slowly or the hard drive spins much more frequently, you may have been recently infected. 

Your computer crashes frequently

Similarly, if your computer starts to experience more frequent crashes, you could be affected by a virus. Often, the crashes aren't intended, but they're a side effect of poorly engineered malware. 

You start seeing random pop-up windows

You shouldn't often see a lot of pop-up windows when using your computer, so if they suddenly start cluttering your browser or desktop, malware is likely to blame. The most common reasons for pop-ups are adware, scareware, and ransomware. 

Your computer is running programs you didn't install

There's a good chance of an infection if you discover new programs running on your PC, and they might insert themselves into the Windows startup process. 

Your email address is sending unwanted emails

Malware is often designed to hijack your email to send unrequested emails to your contacts. Each email probably contains a link or attachment with a copy of the malware.  

How to avoid contracting malware

If you follow basic precautions, remain aware of the risk, and run some form of anti-malware software, you don't have a lot to worry about — malware is much easier to protect against today than it was 10 or 20 years ago because Windows is far, far more secure and the tools to combat malware are much better. To keep yourself safe, follow these tips:

Keep your computer updated

Always be sure that your computer's operating system is up to date with the latest Windows and security updates. 

Install and use anti-malware software

Make sure you use some kind of anti-malware software. This can be the security software built into Windows 10 or a third-party antivirus app. This is critical — the main reason malware is a shadow of its former threat is because PCs are so much better protected today.  

Don't click anything you don't trust

Finally, this advice is just as important today as it was in 1997: Never click anything you don't completely trust. Think about every link you click on, every attachment you open, and every program you download from the internet. If its legitimacy seems questionable, don't take the risk. 

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Dave Johnson

Freelance Writer

Dave Johnson is a technology journalist who writes about consumer tech and how the industry is transforming the speculative world of science fiction into modern-day real life. Dave grew up in New Jersey before entering the Air Force to operate satellites, teach space operations, and do space launch planning. He then spent eight years as a content lead on the Windows team at Microsoft. As a photographer, Dave has photographed wolves in their natural environment; he's also a scuba instructor and co-host of several podcasts. Dave is the author of more than two dozen books and has contributed to many sites and publications including CNET, Forbes, PC World, How To Geek, and Insider.

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