Introduction to Viruses


What is a virus?:
It is a program that can enter a computer in many different ways. These programs, or viruses, are special programs in that they can cause unwanted or damaging effects or take advantage of exploits and operating system weakness to cause damage, system instability, or even allow other programs or users to access your network or data files. When a virus enters a computer or network, it will often try to situate itself in a place where it can be activated and distributed, unintentionally, by the user. A virus will not act until it has been run or until certain pre-established conditions have been met, called the "trigger" condition (a specific date, an operation carried out by the user, etc.). In many cases, the effects produced by a virus, called the "virus payload", will not be seen until some time after it has infected the computer. A typical characteristic of viruses is their capacity to reproduce and spread to other files or programs.


Why are they called "viruses"?:
Computer viruses are called viruses due to their similarities with biological viruses. In the same way as biological viruses enter the body and infect the cells, computer viruses get into the into the computer and infect files. In addition, both types of virus can reproduce themselves and spread, passing the infection from one infected system to others.


The effects produced by a virus can range from catostrophic to simply annoying: they can damage or delete data stored in a computer, cause the infected computer to crash, display on-screen messages, etc.


In addition to propagation and infection techniques, many viruses will also use "evasion" techniques. This means that the virus has techniques or a defense system that makes it difficult to detect it and helps it avoid any action taken against it.


How do I know if I have got a virus?:
It may appear that you have a virus in your computer, but you cannot be sure that this is the case until it is detected using an antivirus tool (programs that detect and eliminate viruses). Some actions that can be carried out by a virus are obvious enough to be recognized and could include: messages displayed on-screen, operations slowing down, the properties of some files change, files and/or folders disappear, the computer will not start, the content of the infected disk is lost, etc.


Viruses are increasingly sent via e-mail, therefore it is important to delete all suspicious and/or unsolicited messages. However, messages known as hoaxes are e-mail messages that inform you about the existence of a possible virus, but are NOT viruses. If you have any doubt about an email or attachment, DO NOT open it.


What do viruses infect?:
The main target of viruses are files located in storage devices such as hard and floppy disks. They target program files, although other types of files and documents can also be infected. A program is simply a file with an .EXE or .COM extension, which can be run to perform specific operations.


As we have already mentioned, there are viruses designed to infect files that are not programs. However, these documents contain elements known as macros. Macros are small programs that the user can include in certain types of files.


Other elements prone to virus attack are the storage devices themselves. By attacking the places in which files are stored, the damage produced by the virus will affect all of the information they contain.


How did viruses arise?:
In the 60s, some scientists developed a game for programmers called CoreWar. The objective of the game was to introduce some programs in memory that made the other programs run certain instructions. The aim was to saturate the memory and for each program to act independently, carrying out certain actions.


Now, however, it is not a game. Some experts believe that over 1,000 new viruses, trojans, exploits and hoaxes are created and released in the world every year. Many of these are simple programs that are easily stopped by most good anti-virus software. These are typically written by amatuer programmers and released as a prank or joke, but have spread further than imagined. A growing number of these, however, are highly sophisticated and cleverly designed programs created by professional "hackers" and are released with the intention to create widespread damage or havoc.


The problem with many of these viruses is that effective prevention, detection and disinfection is often impossible until it has been active for awhile. The "Melissa" virus is a perfect example of how a cleverly written virus can become widespread before effective defensive can be created and distributed.


  


Common Virus Entry Points


The first question that people often ask themselves is: How did this virus get into my computer, or attack it? Knowing the answer to this question can often prevent infection by protecting the possible virus entry points.
Here are the most common entry points used by viruses:


Removable disk drives
Computer networks
Internet


Removable Disk Drives: Disk drives are storage devices on which data is stored in the form of files or documents. These disk drives enable documents to be created on one computer and then used on another. Among these types of storage devices are: floppy disks, CD-ROMs, and Zip and Jaz disks. The last two types are simply special disks with a larger capacity than floppy disks. If any of these are infected, the other computers on which they are used will be infected. E-mail messages can also be stored in these storage devices, which may also be infected.
Floppy disks (or other extractable disks) can store programs, files, Web pages (HTML), e-mail messages with attached files, compressed files, etc. Any of these elements could be infected. Similarly, what is known as the boot sector of the disk could also be infected with a boot virus. Although it still happens, infections produced from floppy disks have decreased significantly to 10%. This means of spreading viruses has given way to much quicker means of propagation, such as e-mail.
Although in general CD-ROM drives can only read the content of a disk but cannot write on them, nowadays it is possible to read and write (record) on a CD-ROM. This, along with the large quantity of information that they can store has led to a large number of infections. In addition, many computers can be booted from a CD-ROM. This option can also lead to an increase in the number of infections.



Computer Networks: A network is a group of interconnected computers that makes it easier for groups of people to work together. Each computer that forms part of the network can connect to all other networked machines. Through this system it is possible to transfer information from one computer to another and/or access the information stored in one of them from the rest. If the information (programs, files, documents, etc.) that is accessed or transferred from one computer to another areinfected, the computers that accessed this computer, or those involved in the transfer, could also be infected.
The network connections can be local and/or remote, allowing computers and laptops to connect via cable, an Intranet, modem, etc. In short, this means that the network can be accessed from several points. You only need to consider a single computer and the means through which a virus can enter it, then multiply this by the computers in the network and mobile machines (such as laptops) that can connect to this network in order to get an idea of the myriad ways through which viruses can enter a network.


Internet: The Internet is becoming an increasingly popular means of obtaining information, sending and receiving files, sending and receiving news, or downloading files. All of these operations are based on transferring information and the interconnection of millions of computers all over the world. This means that as well as data, you may well be receiving a hidden virus. This simple fact alone has allowed virus attacks to grow at an unprecedented rate and has currently made The Internet the biggest virus entry point. Infection via Internet may be produced through a number of different means, including the following:

E-mail
Web pages
File transfers (FTP)
Downloads
Newsgroups


E-mail: Documents and files can be sent and received via e-mail in the form of attachments. These files could be infected. When an e-mail message is opened and the file it contains is run or opened, the computer that has received the message will become infected. The most important characteristics of infection via e-mail are as follows:




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