A computer virus is a type of malicious software (malware) designed to replicate itself and spread from one computer to another, often damaging systems and compromising data in the process. Viruses can attach themselves to legitimate programs or files and are typically spread through email attachments, internet downloads, or infected removable media.

Characteristics of Computer Viruses

1. Self-Replication; A virus can make copies of itself, often without the user's knowledge, allowing it to spread quickly to other files and systems.

2. Host Attachment; A virus attaches itself to a legitimate program or file, executing when the host is run. This makes detection difficult, as the virus is hidden within legitimate software.

3. Activation Trigger; A virus often has a specific trigger that activates its payload, such as a particular date or action by the user. The virus may remain dormant until the trigger conditions are met, making it harder to detect.

4. Payload Delivery; The virus delivers its payload, which can range from displaying annoying messages to deleting files or corrupting data. Causes harm to the infected system, varying from minor annoyances to significant data loss.

Types of Computer Viruses

1. File Infector Viruses

Attach themselves to executable files (.exe, .com) and are activated when the host file is run. Examples: CIH (Chernobyl), Sasser.

2. Macro Viruses

Infect macro-enabled documents (e.g., Microsoft Word, Excel) and execute when the document is opened. Examples: Melissa, Concept.

3. Boot Sector Viruses

Infect the boot sector of a storage device (e.g., hard drive, USB drive) and are activated when the system starts up. Examples: Michelangelo, Stone.

4. Polymorphic Viruses

Change their code or appearance with each infection, making them harder to detect with traditional antivirus software. Examples: Storm Worm, Marburg.

5. Metamorphic Viruses

Rewrite their own code with each infection, creating a new variant and making detection even more challenging. Examples: Simile, ZMist.

6. Resident Viruses

Embed themselves in the system memory and infect files as they are accessed by the system or user. Examples: Randex, CMJ.

7. Non-Resident Viruses

Do not embed themselves in the memory but instead infect files when the infected program is executed. Examples: Cascade, Vienna.

Methods of Infection and Prevention

1. Email Attachments; Viruses are often spread through malicious email attachments. Do not open suspicious emails or attachments from unknown senders.

2. Internet Downloads; Downloading infected files or software from untrusted sources can introduce viruses. Only download software from reputable websites and verify their authenticity.

3. Removable Media; Infected USB drives or external hard drives can spread viruses when connected to a computer. Scan removable media with antivirus software before accessing files.

4. Network Sharing; Viruses can spread through shared network resources or drives. Secure network shares and use strong access controls.

5. Software Vulnerabilities; Exploiting vulnerabilities in software to spread and execute viruses. Keep software and operating systems updated with the latest patches.

Symptoms of Virus Infection

1. Slow System Performance. The computer runs slower than usual as the viruses can consume significant system resources.

2. Frequent Crashes. The system frequently crashes or shows error messages.

This is usually because virus activities can destabilize the operating system.

3. Unusual Behavior. Programs behave unexpectedly or files disappear as viruses corrupt or delete files.

4. Unexpected Pop-ups. Random pop-up messages or ads appear displaying unwanted messages.

5. Increased Network Activity. Unusual or increased network activity as the virus attempts to spread to other systems.

6. Disabled Security Software. Antivirus or firewall settings disable as the virus tries to avoid detection.

Prevention and Protection

1. Install Antivirus Software; Use reputable antivirus software to scan for and remove viruses. It is also important to keep the antivirus software updated to protect against new threats.

2. Regular Software Updates; Keep your operating system and all software updated with the latest security patches. Enable automatic updates where possible.

3. Avoid Suspicious Links and Downloads; Do not click on suspicious links or download files from untrusted sources. Verify the authenticity of the sender and website before downloading anything.

4. Use Strong Passwords; Create strong, unique passwords for all accounts. Use a password manager to keep track of your passwords securely.

5. Enable Firewalls; Use firewalls to block unauthorized access to your computer. Ensure both hardware and software firewalls are enabled.

6. Regular Backups; Regularly back up important data to a secure location. Use both cloud storage and physical backups for redundancy.

7. Educate Users; Educate yourself and others about safe computing practices. Awareness training can help prevent accidental virus infections.

Computer viruses are a significant threat to digital security, capable of causing widespread damage and data loss. Understanding the types of viruses, methods of infection, and preventive measures is crucial for protecting your systems and data. By staying vigilant and employing robust security practices, you can mitigate the risks associated with computer viruses.