how virus spread

First of all we have to know that what is a virus.

What is a virus:

Computer virus is a malicious program that requires a host and is designed to make a system sick, just like a real virus.Virus can spread from computer to computer, and they can replicate themselves.Some viruses are categorized as harmless pranks, while others are far malicious. Broadly three types of viruses are:

1.  File Infectors --  attach themselves to a program file.
2. Boot Sector Virus -- install themselves on the beginning tracks of a hard drive.
3. Macro Viruses -- infect data files.

Most  viruses are spread by e-mail attachment and warn them to be suspicious of any files attached to unsolicited messages. The following are characteristics of a computer virus:

• It is able to replicate.
• It requires a host program as a carrier.
• It is activated by external action.

Its replication ability is limited to the (virtual) system.

How Virus spread:

Computer virus move from computer to computer by attaching themselves to files or boot records of disks and diskettes. These days it is not uncommon to find them in e-mail attachments and other programs that can be downloaded from the internet.

A virus is a relatively passive agent that can travel from one file to another on the same computer if the infected file is executed, from computer memory to a file in disk, on a disk that is carried by from one computer to another (some company prohibit floppy drives, thereby preventing users from copying information onto their computers), on e-mail attachment executable files, and over a modem or network connection.

Damage the viruses cause:

Viruses' main objective is to make your system unstable and cause harm to data. Mainly these cause damage in many ways:

• Can destroy file allocation tables (FAT) and lead to the corruption of an entire file system, resulting in the need to fully reinstall and reload the system.
• Can create bad sectors on the disk, destroying parts of programs and files.
• Can decrease the space on hard disks by duplicating files.
• Can format specific tracks on the disks or format the entire disk.
• Can destroy specific executable files and alter data in data files, causing a loss of integrity in the data.
• Can cause the system to hang so that it does not respond to any keyboard or mouse movements.

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 Read more about Types Of Viruses.