Honestly, most people use the word "virus" to describe anything that makes their laptop act weird. If your browser starts opening random tabs or your files suddenly have weird extensions, you probably tell your friends, "Yeah, I got a virus." But technically? You’re probably wrong. A real virus is just one specific breed of digital parasite in a massive, ugly ecosystem of malware.
Think of it like biology. A flu virus isn't the same thing as a tapeworm, even though both make you feel like garbage. In the world of types of viruses in a computer, the distinctions actually matter because how you get rid of a boot sector infection is a whole different ballgame compared to scrubbing out a resident virus living in your RAM.
The Resident Virus: The Uninvited Houseguest
This one is particularly annoying. A resident virus doesn't just hitch a ride on a file and wait for you to click it. It actually installs itself directly into your computer’s memory (RAM). Because it lives in the memory, it can persist even if you delete the original "infected" file. It just sits there, lurking in the background, messing with every single file or program your operating system tries to run.
It’s like a squatter who changes the locks.
Some versions of the famous Magellan or ** Jerusalem** viruses from back in the day operated on this level. They don't just strike once; they stay. If you’re dealing with a resident virus, you’ll notice your system slowing to a crawl because the virus is constantly intercepting tasks. It’s basically middle-manning your CPU.
Why Direct Action Viruses Are "Old School"
If resident viruses are squatters, direct action viruses are more like a smash-and-grab thief. These are the classic types of viruses in a computer that most people imagine. You download a file—maybe a "free" movie or a cracked game—and you double-click it. The virus executes, does its dirty work (usually infecting other files in the same folder), and then... it kind of just stops. It doesn't stay in your memory. It doesn't hide. It performs its specific function and then becomes inactive.
The Vienna virus is the textbook example here. It would hunt for .com files, destroy them, and then it was done. It’s destructive, sure, but because it doesn’t hide in the RAM, it’s much easier for modern antivirus software like Norton or Bitdefender to spot and kill. You just delete the infected file, and the threat is gone.
The Nightmare of the Boot Sector Virus
This one is a relic from the floppy disk era that has somehow survived into the age of USB drives and shared networks. A boot sector virus targets the Master Boot Record (MBR). This is the very first part of your hard drive that the computer reads when you turn it on.
Imagine the virus grabbing the steering wheel before you’ve even pulled out of the driveway.
Since the virus loads before the operating system even starts, it can be incredibly hard to detect. You might think your Windows or macOS installation is just "broken" because the OS won't even load. Back in the 90s, the Michelangelo virus caused a global panic because it was designed to stay dormant until March 6th—the artist's birthday—and then overwrite the boot sector, effectively turning computers into expensive paperweights.
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- It bypasses the OS.
- It spreads via physical drives (USB sticks).
- It requires a deep-level format or specialized recovery tools to fix.
Multipartite Viruses: The Overachievers
Some viruses are just extra. A multipartite virus is basically a hybrid that uses multiple methods to spread. It might infect your files (like a direct action virus) and your boot sector at the same time. This is a nightmare for IT professionals.
Why? Because if you clean the files but forget the boot sector, the virus just reinfects the files the next time you restart. It’s a loop. The Invader virus was a classic example of this—it would infect the MBR and then start gobbling up files as soon as the computer was running. It’s the "scorched earth" policy of the malware world.
Macro Viruses: Hiding in Your Spreadsheet
You’ve probably seen that warning in Microsoft Word or Excel that says "Enable Macros?"
Do not click that unless you know exactly what it is.
Macro viruses are written in the same macro language used for software applications. They aren't apps themselves; they are snippets of code hidden inside documents. When you open a "Company_Invoice.doc" and enable macros, the virus executes. It can then spread to every other document on your computer or even mail itself to everyone in your Outlook contacts.
The Melissa virus in 1999 used this exact trick. It didn't destroy your hard drive, but it spread so fast by emailing itself to the first 50 people in a user's address book that it actually crashed the email servers of major corporations. It was the first time people realized a "word document" could be a weapon.
Polymorphic and Metamorphic: The Shapeshifters
This is where things get really sci-fi and scary. Most antivirus programs work by looking for a "signature"—a specific string of code that identifies a known virus.
Polymorphic viruses are smarter. Every time they replicate, they change their own code. The function stays the same, but the signature changes. It’s like a criminal who changes their fingerprints and face after every heist.
- Polymorphic: Changes the encryption or the "wrapper" of the code but keeps the core logic.
- Metamorphic: Completely rewrites its own code from scratch every time it moves to a new file.
The Storm Worm was a famous example of this kind of adaptive behavior. It made it nearly impossible for traditional signature-based scanners to keep up. This is exactly why modern security tools now use "heuristics" and AI to look at behavior rather than just signatures. If a program is trying to encrypt your entire drive, it doesn't matter what its code looks like—it’s bad news.
The Fileless Virus Trend
Okay, technically, many experts argue if this belongs in a list of types of viruses in a computer, but you need to know about it. A fileless virus doesn't live in a file. It doesn't need you to download an .exe. Instead, it uses legitimate programs already on your computer—like PowerShell or Command Prompt—to execute malicious commands.
Because there is no "malicious file" to scan, your antivirus might not see anything wrong. It just looks like Windows is doing Windows things. According to reports from cybersecurity firms like CrowdStrike, fileless attacks have spiked by over 70% in recent years because they are so effective at bypassing traditional defenses.
Why Does This Keep Happening?
It’s easy to think we’d be past this by now. But the truth is, humans are the weakest link. Most of these types of viruses in a computer require a "host" or a "trigger." That trigger is usually you. Whether it's clicking a link in a phishing email or plugging in a random USB drive you found in a parking lot (please, never do this), the virus needs an entry point.
We also have the "Internet of Things" (IoT) problem. Your smart fridge or your Wi-Fi-connected lightbulbs probably have terrible security. Hackers aren't just looking for your laptop anymore; they want any device with a processor and an internet connection.
Actionable Steps to Protect Yourself
Stop worrying about "getting a virus" and start building a defense system. It's not about being lucky; it's about being annoying to hack.
1. Use a Non-Admin Account for Daily Tasks
Most people run their computers on an Administrator account. If a virus hits an Admin account, it has the keys to the kingdom. If you use a "Standard" user account for your daily browsing, many viruses won't have the permissions they need to install themselves into the system folders.
2. The "3-2-1" Backup Rule
Viruses can be deleted. Data loss is forever.
- 3 copies of your data.
- 2 different media types (e.g., cloud and external hard drive).
- 1 copy off-site (in case of fire or theft).
3. Update Your Firmware, Not Just Your Apps
We all update our apps, but when was the last time you updated your router’s firmware? Or your BIOS? Boot sector viruses love outdated hardware interfaces. Check your manufacturer's website once every six months.
4. Kill the Macros
Go into your Office settings (Word/Excel) and set your macro settings to "Disable all macros with notification." If a document asks you to enable them, and it’s not something you specifically created, close the file and delete it immediately.
5. Heuristic-Based Security
Ensure your security software isn't just a "scanner." You want something that offers "Behavioral Analysis." This is the only way to catch the polymorphic and fileless threats that don't have a known signature yet.
The landscape of types of viruses in a computer is constantly shifting. Yesterday it was floppy disks; today it's "living off the land" with fileless scripts. The tech changes, but the goal is always the same: gaining control. Stay skeptical of every "urgent" notification and every "free" download. Usually, if it’s free, you—or your data—are the price.