A computer virus infects files by attaching its malicious code to them, modifying the original file to include the virus. When the infected file is executed, the virus code runs first, allowing it to spread to other files.
The Infection Process Explained:
Here's a breakdown of how a computer virus infects files:
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Attachment: The virus code attaches itself to a host file. This host file is usually an executable program (.exe), a document (.doc, .pdf), or a script file.
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Modification: The virus modifies the host file. This involves inserting the virus's code into the host file, often by overwriting or appending to it. The virus may also replace the beginning of the host file with its own code, redirecting the original program to the virus first.
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Activation: When a user executes the infected file, the virus code is executed before the original program.
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Replication and Spreading: Once activated, the virus replicates itself by finding other susceptible files on the system or network. It repeats steps 1-3, infecting more files. This is how the virus spreads.
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Payload Delivery (Optional): Some viruses, after replicating, deliver a "payload." This can range from displaying annoying messages to stealing data, corrupting files, or giving the attacker remote control of the system.
Methods of Infection:
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Executable Files: Viruses often infect executable files (.exe) because these files are directly executed by the operating system.
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Document Files with Macros: Microsoft Office documents (like Word and Excel files) can contain macros (small programs). Viruses can be embedded in these macros, and when the user opens the document and enables macros, the virus runs.
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Script Files: Scripting languages like JavaScript and VBScript can also be used to spread viruses. Web pages containing malicious JavaScript can execute automatically when visited, leading to infection.
Example Scenario:
Imagine you download a seemingly legitimate game from an untrusted website. Unbeknownst to you, this game's executable file (.exe) has been infected with a virus. When you run the game, the virus activates. The virus might then search your computer for other .exe files, attach itself to them, and modify them. The next time you run one of those infected programs, the virus will spread further.
In summary, computer viruses infect files by modifying them to include the virus's code. When the infected file is executed, the virus runs, replicates itself, and spreads to other files, potentially delivering a harmful payload.
