Keyloggers invade PCs (and Macs, and Androids, and iPhones) in the same way that other malware does. They install when you click on a file attachment that you’ve been duped into opening—most commonly because you fell for a social engineering scheme or a cleverly designed phishing expedition. The attachments can come to you by email, through a text message, an instant message, on social networks, or even through a visit to an otherwise legitimate but infected website, which exploits a vulnerability in it and drops a drive-by malware download. Also, keyloggers rarely arrive solo. The same Trojan that delivers the keylogger can slip other malware on your system—such as adware, spyware, ransomware, or a virus.