A malware infection occurs when malicious code executes on a device or enters an environment, enabling unauthorized actions such as persistence, data theft, encryption, or further compromise. The term commonly covers viruses, worms, trojans, spyware, and similar malware; an infection may begin through a malicious attachment, exploit, drive-by download, removable media, or stolen credentials. Its effects depend on the malware and the privileges of the affected account, and an infected host does not necessarily spread automatically.
For security practitioners, the key concerns are identifying affected hosts, determining the initial access and scope, and preventing lateral movement. Useful controls include timely vulnerability remediation, email and web filtering, application controls, least-privilege accounts, and endpoint monitoring for unusual processes, persistence, or network connections. When infection is suspected, isolate the system without destroying evidence, investigate related accounts and devices, revoke exposed credentials, remove or rebuild the malware, and validate that restored systems are clean.