Computer viruses are programs that infect computers without the user's permission or knowledge. They can replicate themselves and spread from computer to computer. Common symptoms of a virus infection include unexpected messages, deleted files, added files, or issues with hardware like keyboards. A worm is similar to a virus but does not attach itself to other programs; it uses networks to spread. There are different types of viruses classified by how they infect systems, like memory-resident viruses that remain active after infection or boot sector viruses that target the boot process. Macro viruses embed themselves in documents to infect users opening files, while email viruses are aware of email functions and can harvest addresses to send themselves to new users.

1. Computer Virus

2. Computer Virus:
• A computer virus is a program or piece of code that is loaded onto our
computer without our knowledge and runs against our wishes.
• It is a harmful software program
• It can replicate itself
• All computer viruses are man made. Once created and released, however
,their spread is not directly under human control.
• According to Webster’s collegiate Dictionary
Computer virus is “a computer program usually hidden within another
seemingly innocuous program that produces copies of itself and inserts them
into other programs or files ,and that usually perform a malicious action “

3. Common virus infection symptoms
• Unexplained messages appears on the screen
• Specific files are mysteriously deleted
• Unknown files are added to the disk
• An entire disk or drive is erased
• The keyboard doesn’t work properly

4. Worm
• A worm is a special type of virus that can replicate itself and use
memory, but cannot attach itself to other programs.
• It uses a computer network to spread itself

5. Virus types:
Which can be classified according to their origin
techniques, types of files they infect, platform they attack etc
1.Memory resident virus vs non-memory resident virus
A memory-resident virus installs itself as part of the operating
system when executed, after which it remains in RAM from the time
the computer is booted up to when it is shut down. Resident viruses
overwrite interrupt handling code or other functions, and when the
operating system attempts to access the target file or disk sector, the
virus code intercepts the request and redirects the control flow to the
replication module, infecting the target. In contrast, a non-memory-
resident virus (or "non-resident virus"), when executed, scans the disk
for targets, infects them, and then exits (i.e. it does not remain in
memory after it is done executing).

6. Boot sector virus
• Boot sector viruses specifically target the boot sector and/or
the Master Boot Record (MBR) of the host's hard drive or removable
storage media (flash drives, floppy disks, etc.)

7. Macro Virus
•
Many common applications, such as Microsoft Outlook and Microsoft
Word, allow macro programs to be embedded in documents or
emails, so that the programs may be run automatically when the
document is opened. A macro virus (or "document virus") is a virus
that is written in a macro language, and embedded into these
documents so that when users open the file, the virus code is
executed, and can infect the user's computer.

8. Email virus
• Email virus – A virus that intentionally, rather than accidentally, uses
the email system to spread. While virus infected files may be
accidentally sent as email attachments, email viruses are aware of
email system functions. They generally target a specific type of email
system (Microsoft's Outlook is the most commonly used), harvest
email addresses from various sources, and may append copies of
themselves to all email sent, or may generate email messages
containing copies of themselves as attachments.