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Computer viruses used to be a huge problem in the IT world and if you didn't have antivirus software, you'd essentially be opening yourself up to attack.
Fortunately, most modern Operating Systems come with built-in antivirus which deals with most pesky viruses.
Computer viruses are often referred to as "malware", which is short for "malicious software" and there are several different types depending on what they do.
Ransomware
Ransomware locks down a computer system or the files stored on it and demands a payment to be made to the attackers in order to release them.
Usually targeted at large companies who have the money to pay the ransom and a lot to lose if they don't.
An example of this would be Wannacry.
Trojans
Trojans are viruses that disguise themselves as something else, usually to trick antivirus software.
An example of this would be when you download a tampered application from a shady website or trust an unsafe macro in a Microsoft Office document.
Adware
Adware installs advertisements on your computer and collects various data about you to show you more relevant adverts that are more likely to make you click on them.
Browser extensions and toolbars are often used to facilitate this.
Spyware
Spyware is a type of malware which spies on you through your computer.
Either by obtaining your data such as browsing history, credit card information, passwords and other account details or through connected peripherals such as the camera or microphone.
Keyloggers
Keyloggers monitor the keyboard when you are typing and collect all the keystrokes you press, which get sent back to the hackers.
This enables attackers to obtain personal data about you such as account details or credit card information.
Worms
Worms are viruses that reproduce themselves through a network of connected computers, allowing them to spread more rapidly and cause further havoc.
These can sometimes change their form, making them quite difficult to detect by antivirus.
Rootkits
Rootkits allow unauthorised access to your computer without detection, usually remotely.
This allows the attacker to access and control your computer without you even knowing.