There are a few reasons why computers get programs a user didn't mean to install. One common reason is because of regular programs that have software bugs. Malware can use bugs, such as a buffer overflow, to make a program do something it was not designed to do. Malware can also get onto a computer if it tricks a user into putting it there themselves. This can happen when a user plugs in a USB flash drive that has a virus on it already. Malware also commonly uses social engineering to get users to run it, like pretending to be an important email attachment for work. Some malware even pretends to be an anti-malware program to get people to run it.