If a floppy disk, with a virus-infected program is loaded onto another computer, the virus starts spreading very fast. Also any program that the contents is not known and is being downloaded could be a virus. Viruses travel fast using both these ways. The spreading part is the "infection" phase of the virus. Viruses wouldn't be so bad if all they did was replicate themselves, but they don’t. Unfortunately, most viruses also have some sort of destructive "attack" phase where they do some damage. Some sort of trigger will activate the attack phase, and the virus will then "do something" -- anything from printing a message on the screen to deleting all of your data. The trigger might be a specific date, or the number of times the virus has been replicated, or something similar.
In general, most viruses are not very threatening any more. The first reason for the decline has been the huge size of today's programs. Nearly every program you buy today comes on a compact disc (CD). Compact discs cannot be modified, and that makes viral infection of a CD impossible. The programs are so big that the only easy way to move them around is to buy the CD. People don’t carry applications around on a floppy disk like they used to, when floppy disks were the main way of transferring information and programs
The latest thing is the e-mail virus, and the Melissa virus in March of 1999 was spectacular. Melissa spread in Microsoft Word documents sent via e-mail, and it worked like this. Someone created the virus as a Word document uploaded to an Internet newsgroup. Anyone who downloaded the document and opened it would trigger the virus. The virus would then send the document (and therefore itself) in an e-mail message to the first 50 people in the person's address book. The e-mail message contained a friendly note that included the person's name, so the recipient would open the document thinking it was harmless. The virus would then create 50 new messages from the recipient's machine. As a result, the Melissa virus was the fastest-spreading virus ever seen! As mentioned earlier, it forced a number of large companies to shut down their e-mail systems.
The ILOVEYOU virus, which appeared on May 4, 2000, was even simpler. It contained a piece of code as an attachment. People who double clicked on the attachment allowed the code to execute. The code sent copies of itself to everyone in the victim's address book and then started corrupting files on the victim's machine. This is as simple as a virus can get. It is really more of a trojan horse distributed by e-mail than it is a virus.
The Melissa virus took advantage of the programming language built into Microsoft Word called VBA, or Visual Basic for Applications. It is a complete programming language and it can be programmed to do things like modify files and send e-mail messages. It also has a useful but dangerous auto-execute feature. A programmer can insert a program into a document that runs instantly whenever the document is opened. This is how the Melissa virus was programmed. Anyone who opened a document infected with Melissa would immediately activate the virus. It would send the 50 e-mails, and then infect a central file called NORMAL.DOT so that any file saved later would also contain the virus! It created a huge mess.
Microsoft applications have a feature called Macro Virus Protection built in to them to prevent this sort of thing. If you turn Macro Virus Protection on, then the auto-execute feature is disabled. By default the option is ON. So when a document tries to auto-execute viral code, a dialog pops up warning the user. Unfortunately, many people don't know what macros or macro viruses are, and when they see the dialog they ignore it. So the virus runs anyway. Many other people turn off the protection mechanism. So the Melissa virus spread despite the safeguards in place to prevent it.
In the case of the ILOVEYOU virus, the whole thing was human-powered. If a person double-clicked on the program that came as an attachment, then the program ran and did its thing.