Computer Virus: An Overview

The world of computing is increasingly exposed to electronic threats in the form of computer virus. These programs can disrupt normal operation, cause data loss, and spread rapidly through individual machines and networks. Below is a concise, updated rewrite covering the history, how viruses work, their types, the harm they cause, and basic prevention.

Introduction and history

The idea of a self-replicating program goes back to John von Neumann’s 1949 work on automata. Early experiments and games explored self-replicating or destructive code. One of the first widely noticed real-world examples appeared in 1985: the “Brain” virus (also called the Pakistani virus), which was embedded in inexpensive software distributed from Lahore. It replicated itself across disks and infected PCs of users who ran that software.

Since then, self-replicating code has evolved from academic curiosities and pranks into a serious security problem that threatens data integrity and system availability.

What a virus is and how it spreads

A computer virus is a program or code that can copy itself and attach to other programs, files, or boot sectors. Like biological viruses, which hijack a cell’s machinery to reproduce, a computer virus uses a host system’s resources to propagate. Many viruses remain inactive until a specific event (boot, execution of an infected program, a scheduled date) triggers them.

Common infection vectors include:

• Booting from an infected disk or media.
• Running an infected executable.
• Exploiting vulnerabilities to install code.
• Spreading across networks and shared drives.

Viruses, worms, and trojans

The term “virus” is often used broadly to cover several categories of malicious code, but they differ:

• Virus: Attaches to files or boot sectors and requires some user action (e.g., running an infected program) to propagate.
• Worm: Self-replicates and spreads across networks without needing to attach to other files or require user action.
• Trojan (Trojan horse): Disguised as a legitimate program that performs malicious actions when run. Trojans do not self-replicate.

Typical damage caused by malware

Beyond replication, malware commonly causes:

• Data corruption or deletion (targeting file contents or critical structures).
• Modification of system structures (for example, changing interrupt or system vectors so the malicious code runs first).
• Rendering files or disks unusable even if the malware isn’t explicitly programmed to destroy data.
• Network disruption and contagion across LANs.

How infections work (basic technical view)

On boot, a PC runs ROM-based initialization (POST), then loads the boot sector and operating system files (historically IBMBIO.COM, IBMDOS.COM, COMMAND.COM on MS‑DOS). If the boot sector or an essential system file is infected, malicious code may be loaded into memory early in the boot process. Many viruses hook into the system by altering interrupt vectors or other system tables so that system calls are routed through virus code before the intended routine runs. Once resident in memory, the virus can intercept I/O, infect additional files or media, and spread.

Classification of viruses

Viruses are commonly classified by their primary infection target or technique:

Boot-sector infectors

• Reside in the disk’s boot sector and load during the boot process.
• Can remain resident in memory and infect other media even after a soft reboot.
• May hide their presence by preserving or relocating the original boot record so the system still appears to boot normally.
• Often leave “bad sectors” or damage the boot area, which can prevent normal booting or access to a disk.

System (memory-resident) infectors

• Attach to system files or memory-resident programs (for example, important command or kernel files).
• Gain control after the system has booted and can manipulate system behavior, error messages, or activate at certain times.
• Stay resident in memory, allowing them to intercept system calls and infect programs as they run.

File (executable) infectors (COM/EXE infectors)

• Attach to executable files (.COM, .EXE historically) by modifying their entry point so execution jumps to the virus code, then returns to the original program.
• Are particularly dangerous because they can spread to many programs, increase file sizes, and make executables unusable if multiply infected or too large to load.

Prevention and mitigation (practical steps)

• Keep systems and software up to date with security patches.
• Run reputable antivirus/antimalware software and keep its definitions current.
• Avoid running unknown or untrusted executables and be cautious with email attachments and removable media.
• Restrict user privileges—limit administrative rights to reduce the impact of infections.
• Use firewalls, network segmentation, and strong authentication to limit worm propagation across networks.
• Maintain regular, tested backups stored offline or on immutable media so you can recover if data is corrupted.
• Educate users: many infections rely on social engineering or careless actions.

Conclusion

Computer viruses and related malware remain a major information-security threat. Understanding how they work—their methods of infection, the damage they can do, and their classifications—helps in designing effective defenses. Combining technical controls (antivirus, patching, backups, access control) with user training is the most reliable approach to reducing the risk and impact of these threats.