Malware has been a fairly big interest of mine since I was a lot younger. When I was a kid on the internet, I often downloaded programs packed with adware, spyware, and other kinds of malware. This lead to me developing a big fear of them (a significant amount of my nightmares involved/still involve destructive malware), as well as a fascination with them. In my earlier teens, I used to code very inefficient malware on older computers 
where it was safe and I couldn't cause any real damage. It was fun! While I have no use for these skills anymore, I still find older malware very interesting. Obviously, I do not endorse malware nor think it's a good thing, I simply find it intriguing.
Malware has pretty much existed since the dawn of computing. The title of 'first computer virus' is generally attributed to Creeper, a program which displayed a small, harmless message to a user, then moved itself to the next computer through the ARPANET. Later, a different variant was made to replicate itself instead of simply moving. 
Creeper was created in 1971 as a simple experiment, but quickly laid the groundwork for future computer viruses.
The Morris Worm is a particularly infamous computer worm which became one of the oldest forms of malware to spread through the internet. The source code for the Morris Worm is still stored today in a floppy disk at the Computer History Museum in Mountain View, California. Over time, computer viruses became much more complex and dangerous. Certain malware such as MyDoom, ILOVEYOU, and Slammer, gained mass media attention and are still well-known today.
The golden age of computer viruses was in the late 90s and early 2000s, a peak time of virus development and spread. Post-Y2K, people became more educated in cybersecurity, and malware had to become more cryptic in order to adapt. This remains the case today, with relatively non-destructive adware and spyware being the most common forms of malware we still see.
Malware exists in many different forms. Some of the more commmon forms include...
Adware are a type of malware that display unwanted advertisements on a user's computer. Typically, these ads are shown in the form of pop-up windows, which are sometimes uncloseable or injected into webpages. Adware is one of the least dangerous types of malware, some forms being considered 'grayware' rather than malware. They typically exist only to generate profit for the adware's developer. BonziBuddy is a well-known piece of adware, and is frequently memed.
Backdoors are programs that allow an individual or an organization to slip by security systems pre-installed within a computer and gain access to the system. Backdoors can be used to inject malware (often spyware) without being detected. Backdoors can be used directly by another person to access important passwords and files, and destroy files that don't have backups. Backdoors are often used by government organizations. WannaCry used security vulnerabilities and backdoors to infect over 300,000 computers.
Ransomware is a form of malware that encrypts a user's files, then demands payment from the user to recover their files (therefore holding their files ransom). Ransomware sometimes doesn't allow a user to recover their files regardless of whether or not the ransom is paid. CryptoLocker was a ransomware attack that infected around 250,000 PCs, and earned an estimated 27 million US dollars in it's time.
Rootkits are malicious software intended to cryptically access to a device's administrator privileges while disguising it's own files, or the files of associated malware. They are often very difficult to detect, as they actively try to circumvent efforts to discover them. Rootkits are often used to spy on or access private user information.
Spyware is a program that monitors a user's activity, and sends the information to a third party. They can be used to steal sensitive information, like identity information and bank accounts, or to sell data to targeted advertisers.
Trojans, also called trojan horses, are a kind of malware that deceives a user by pretending to be something innocuous, or even helpful, to bait a user into installing it. Trojan horses often disguise themselves as anti-malware programs, or harmless email files. The trojan horse program 'Zeus' was spread via email, and would steal online bank account data from those infected.
The term 'computer virus' is often used to refer to all kinds of malware, though not all malware are computer viruses. Computer viruses are malware that repeatedly replicate to spread themselves to other computers or to files within a computer, with the intent of causing harm. Due to their ability to conceal themselves within other programs, they can spread very rapidly. They're differentiated from computer worms because they require user or program interaction in order to spread, while computer worms can replicate and spread all on their own.
Computer worms are similar to a virus, in that they also spread and replicate themselves rapidly, but are able to self-replicate without the input of a host program or a user. They can travel to other computers through a variety of means, often using computer networks or even emails. Computer worms can spread very quickly, but are not always intended to cause harm to a host computer. Their quick replication can overwhelm and disrupt important networks, like those belonging to companies or governments. The ILOVEYOU worm is one of the most well-known viruses in history, which affected ten million Windows computers in May of 2000.
My favourite piece of computer malware is Kuku. It's a computer virus made in eastern Europe made to infect PCs running MS-DOS. It injects itself within .com files. Upon a file infected by Kuku being ran, it displays several colourful, sometimes flashing boxes with the text "Kuku!". While the payload runs, it disables all keyboard inputs aside from CTRL+ALT+DEL. The payload will continue progressing until it fills the entire screen. It's my personal favourite due to the spread method, as well as the interesting payload. The meaning of "kuku" here is likely from the Slavic "ку-ку", a sound that mimics a cuckoo bird and is similar in meaning to the English "gotcha!".
Below is a demonstration video of Kuku, made by the wonderful danooct1. His channel has a lot of other demonstrations of older (and some newer) malware. I highly recommend you check him out!
This next section is just a pet peeve of mine surrounding 'YouAreAnIdiot'. YouAreAnIdiot is not, and has never been, harmful malware! Simple Javascript prank websites were fairly common in the past, for example websites that'd crank your volume and play an embarrassing audio, and YouAreAnIdiot is simply a variant of that trend. It did not cause any lasting harm to the computer, and could be easily stopped by either closing the script in task manager or resetting the PC. I still don't recommend messing around with modern equivalents of it, but the idea that it is some sophisticated or dangerous malware has always bothered me.
