Key malware types:
Virus (requires user to run infected file),
Worm (self-propagating via network),
Trojan (disguised as legitimate),
Ransomware (encrypts files for payment),
Rootkit (hides malware, OS-level),
Fileless (lives in memory, no disk files).
Know each type and how they spread/persist. Fileless malware is hardest to detect (no files for AV to scan). Rootkits are hardest to remediate (compromise OS integrity). Ransomware is the most financially damaging. Worms spread the fastest (no user interaction needed).