Malicious computer software is almost as old as the personal computer itself.
Although it’s often associated with PCs, the first ever recorded computer virus targeted Apple computers in 1982.
As such, it predated the first PC virus by four years.
Since then, a combination of secure operating systems and small market share has seen Apple devices attracting less unwanted attention than PCs, which dominated computing for decades.
Windows is the most attacked operating system in human history, and new threats are being created all the time by criminals and those of a bloody-minded disposition.
After all, 1982’s ground-breaking Elk Cloner Apple virus was created by a bored 15-year old, for sheer devilment.
More than just a virus
Today, viruses are a subset of malicious software, known collectively as malware.
Malware may be intended to spy on user activities, impersonate the victim for fraudulent purposes, or seize control of a device entirely.
Common types of computer malware include ransomware, spyware, adware and Trojans.
It can be hard to tell these terms apart, or identify a particular strain of malware as it wreaks havoc.
And that’s important, because recognising errant software often simplifies the process of removing it, allowing normal life to resume.
Malware symptoms include very slow device performance, strange icons appearing on the desktop, pop-up windows which can’t be closed and problems connecting to the internet.
Below, we consider the main types of computer malware.
1. Viruses. Where else to start than with the oldest and most pernicious malware variant?
A virus is a self-replicating piece of code capable of infecting other files it comes into contact with, and it’s typically distributed as an email attachment.
Viruses are often used to drag devices into botnets – huge networks of enslaved computers used for mass-processing tasks like Bitcoin mining or denial-of-service web server attacks.
2. Worms. Think of these as turbocharged viruses, whose main aim is self-replication. Unlike viruses, they don’t need to be activated by opening an infected file.
The simple act of self-replication can destroy everything from individual devices to whole networks, potentially co-opting a device’s entire system resources in the process.
As with viruses, worms are good at bringing down servers and forcing websites offline. However, antivirus software is constantly being updated to block and destroy new worms.
3. Trojans. The Greek myth about the Trojan Horse provides a fitting title for seemingly legitimate programs which are incubating an unwelcome surprise.
Trojans rely on a user unwittingly opening or executing a file, at which point they can give criminals remote access to files and folders stored on a device’s hard drive.
Trojans are often designed to steal personal data like bank account logins and credit/debit card details, through keystroke monitoring, screen mirroring or simply via file access.
4. Ransomware. This is the most frightening form of malware, because it involves a criminal remotely taking control of a device and encrypting its data.
The victim is prevented from accessing their own data unless they pay a ransom – usually in an untraceable cryptocurrency like Bitcoin.
An estimated 30 per cent of people who pay up still don’t get their data back. Worse, the tools used to seize control in the first place are probably still in situ, for future attacks.
5. Adware. This is less damaging than other forms of malware, but is hugely annoying nonetheless.
Adware is software which bombards a device with pop-up adverts. It can hijack web browsers, constantly redirecting them to low-quality websites advertising dubious products.
Adware may be immensely embarrassing. Imagine if your works PC suddenly started flashing up pornographic pictures, with no way of stopping it or closing the pop-up windows.
6. Spyware. Once installed or downloaded (typically as a Trojan), spyware hides on your system, monitoring keystrokes and data transfers.
It enables criminals to acquire your online login credentials, allowing them to impersonate you and potentially access your financial accounts.
Spyware is also deployed by jealous partners and paranoid bosses, though its executable files are generally easy to remove – something it shares with adware.