Some careless or unaware users click on things that install malware on their computer.
The malware then gets all the address from their email and sends out spam laced with malware links to install the same thing on their friends computer.
I think it is best to use Yahoo email or similar free email services, install free malwarebytes on your computer and never click on anything suspicious, email or things that pop up, to find out what it is.
Google: Malware
The term 'malware" covers all sorts of malicious software designed to harm a computer or network. Malware can be installed on your machine without your knowledge, often through deceptive links or downloads posing as something you might be interested in. Once malware has been installed on your computer, cyber criminals can sometimes try to access your personal information. They do this by logging your keystrokes or monitoring your computer's activity. Your computer could also be controlled and forced to visit websites, send spam email or perform other actions without your knowledge. The effects of malware can be anything from a brief annoyance to identity theft.
A few examples of malware:
virus: a computer program that can copy itself and infect a computer. worm: a self-replicating malware computer program, which uses a computer network to send copies of itself to other computers on the network. spyware: a malware that collects small pieces of information about users without their knowledge. adware: any software package which automatically plays, displays, or downloads advertisements to a computer. Trojan horse: a destructive program that masquerades as an application. The software initially appears to perform a desirable function for the user prior to installation, but steals information or harms the system.......