Malware is short for malicious software, the types of programs that threaten security and privacy. Commonly known malware include computer viruses, worms, and trojans.

You may see “virus” used as an umbrella term, since many malicious programs combine traits of multiple types of malware. Still, each type of malware has unique behavior, and the level of threat they pose to data security and user privacy differs. So how can you tell them apart?

Viruses Infect Other Files

Similar to biological viruses, like chickenpox, this type of malware infects files throughout your computer. You see the symptoms almost as soon as your device becomes infected. You may notice your laptop slowing down or files with certain extension names getting deleted. Sometimes, you may see unfamiliar apps installed on your device or otherwise small files ballooning, causing your machine to run out of storage space.

Computer virus infections are difficult to treat or remove entirely, just like their biological relatives. There’s always a good chance that when you attempt to remove a virus, the program will copy itself into a clean file and become dormant—a behavior known as file persistence. In many cases, the most effective way to handle a computer virus is to wipe the infected drive clean, restore your device to factory settings, or even reinstall the operating system.

Going forward, you will also need to make sure you don’t get infected again. Restrict sharing hard drives, don’t plug unknown drives into your computer, and avoid downloading files from untrusted sources.

If you have a Windows computer, consider adjusting your AutoPlay settings for connected Bluetooth devices, CDs, and USB devices. On Windows 11, you can do this by going to Settings > Bluetooth & devices > AutoPlay > Toggle Off. This way, you won't inadvertently copy infected files when someone slots a disc or plugs a drive into your computer.

Worms Copy Themselves

photo of a gummy worm

Unlike computer viruses, worms don’t infect existing files. Instead, they copy themselves and spread between computers on public and private networks. Generally, worms infect computers through unpatched vulnerabilities, but they can also exploit misconfigured security settings or compromised passwords.

Once inside a system, computer worms often spread online via email. For example, email worms self-replicate and send copies of themselves to every contact on the infected computer’s address book. Recipients then receive an email with an attachment or link from a trusted source. When the recipient opens the attachment or infected link, the malware executes its payload, and the cycle repeats. Besides email, worms can also spread via social media, cloud storage, network ports, removable drives, and file-sharing protocols.

Like viruses, worms are tricky to deal with, so it’s better to avoid the infection in the first place. Turn on Microsoft Defender, avoid sharing USB drives, install software updates regularly, and scan email attachments before opening them. Avoid using your private email to sign up for newsletters on unsecured websites, and don’t open email from unknown or untrusted senders.

Trojans Disguise as Gifts

Photo of a Trojan Horse

You may have learned not to look a gift horse in the mouth, but you may want to put on your monocle when it comes to software that looks too good to be true.

Trojans are malware disguised as legitimate software and have become common with the advent of digital piracy. When you download and install a trojan, it works like you would expect (at least initially). But at the same time, the trojan executes its payload. A remote access trojan may open a backdoor to your computer or download additional malware in the background. The malware can also covertly collect your passwords, log keystrokes, and send sensitive information to hackers.

Unlike computer viruses and worms, trojans need users to manually—and unwittingly—execute them. They also don’t copy and spread themselves autonomously. Still, that doesn’t mean you should take them lightly, especially if you download pirated media and software. As a rule of thumb, scan files with an antivirus before opening them to detect and remove trojans.

Other Malware Threats You Should Know About

Picture of threat notification from Microsoft Defender

Viruses, worms, and trojans receive attention because of the scale of their threat. Pesky viruses and worms can cripple regional digital infrastructure within minutes. Trojans can commandeer computers as dirty servers for illegal content, potentially putting you in legal trouble. Yet lesser-known malware that flies under the radar can be just as dangerous.

Scareware Use Fear

Scareware, also known as crimeware, is a type of malware that uses fear-mongering or social engineering tactics to get victims. The common ones appear as fake antivirus ads, low phone storage alerts, or offers to optimize your battery health when you visit a web page. If you believe the alert and download the fake program, it will perform an elaborate, fake scan and inform you that there’s something wrong with your device. You will then be nudged to pay for full access, which is a ploy to collect your credit card information.

Spyware Spies on You

Pomeranian working on an iPad

Spyware includes everything from "keyloggers" that log your keystrokes to steal your banking details to advertising programs that monitor your web browsing activity. Advanced spyware can even access your microphone and camera without you knowing.

Spyware is generally designed to make money for its creators through data collection or conduct blackmail leveraging sensitive files stolen from the victim’s device.

Ransomware Lock Out and Extort

This type of malware is made for extortion. Once they infect a computer or network, they encrypt the files and lock out the users. The program then displays a message demanding a ransom in exchange for the unlock key.

Ransomware is usually used for targeted attacks on victims most likely to pay the ransom, like hospitals, schools, local governments, and businesses. These entities make good targets because such incidents often make the news, putting additional pressure on victims to fix the situation before they lose clients’ trust. In addition, these victims handle a lot of data and may not have yet backed up the latest copy of every file.

Most Malware Are Pocket Knives

Swiss Knives Lying on Floor

Most of the malware you will see or hear about are a combination of the aforementioned types of malware. A trojan could carry a virus or worm in its payload and vice versa. For example, CryptoLocker is a worm and ransomware: it spreads via email and encrypts files on the infected computer until the owner pays for a decryption key. You may have also heard of Sality, a complex malware with virus, trojan, and worm characteristics. It infects Windows files with .exe or .scr extension names, replicates, and sends copies of itself from the victim’s address book.

On the bright side, because most malware works like a pocket knife, you can prevent infection with similar cybersecurity practices. For starters, ensure that your operating system is always up-to-date. This way, you get the latest patch that closes vulnerabilities before attackers exploit them. In addition, you should have an antivirus on your device. If you use a Windows laptop, you don’t have to download or spend money on one. You can use Microsoft Defender to provide your PC with some protection.

It’s Easier to Prevent Malware Than Remove Them

Ransomware is exorbitant, viruses render files useless, and worms are difficult to remove. Seeing how troublesome malware can be, we recommend avoiding them in the first place. Start by watching how files get on your computer, online or offline. You should also follow cybersecurity best practices.