What is the oldest mode of malware infection?

The oldest known mode of malware infection is infected floppy disks. In the early days of computing, malware was spread by physically sharing floppy disks that contained malicious code. One of the first known examples is the Brain virus from 1986, which spread via floppy disks and infected the boot sector of MS-DOS systems. While outdated today, it laid the foundation for modern malware techniques that evolved into email, web-based, and network-driven attacks.