$course$/Default for IGCSE Theory Computer Viruses <p>A virus is a small piece of {1:MULTICHOICE:Reproduce#wrong answer~Possible#wrong answer~Damage#Wrong answer~Devastating#Wrong answer~Network#Wrong answer~=Software#Correct~E-mail#Wrong answer~Pass#Wrong Answer~Mailing#Wrong Answer~Spreadsheet#Wrong answer~Holes#WrongAnswer~Game#Wrong Answer} that attaches itself onto real programs. For example, a virus might attach itself to a program such as a {1:MULTICHOICE:Reproduce#wrong answer~Possible#wrong answer~Damage#Wrong answer~Devastating#Wrong answer~Network#Wrong answer~Software#Wrong Answer~E-mail#Wrong answer~Pass#Wrong Answer~Mailing#Wrong Answer~=Spreadsheet#Correct answer~Holes#WrongAnswer~Game#Wrong Answer} program. Each time the spreadsheet program runs, the virus runs, too, and it has the chance to {1:MULTICHOICE:=Reproduce#Correct answer~Possible#wrong answer~Damage#Wrong answer~Devastating#Wrong answer~Network#Wrong answer~Software#Wrong Answer~E-mail#Wrong answer~Pass#Wrong Answer~Mailing#Wrong Answer~Spreadsheet#Wrong answer~Holes#WrongAnswer~Game#Wrong Answer} (by attaching to other programs) or wreak havoc. </p> <p>Computer viruses {1:MULTICHOICE:Reproduce#wrong answer~Possible#wrong answer~Damage#Wrong answer~Devastating#Wrong answer~Network#Wrong answer~Software#Wrong Answer~E-mail#Wrong answer~=Pass#Correct Answer~Mailing#Wrong Answer~Spreadsheet#Wrong answer~Holes#WrongAnswer~Game#Wrong Answer}  from computer to computer in much the same way that viruses pass from person to person – their aim is to reproduce and spread as far as {1:MULTICHOICE:Reproduce#wrong answer~=Possible#Correct answer~Damage#Wrong answer~Devastating#Wrong answer~Network#Wrong answer~Software#Wrong Answer~E-mail#Wrong answer~Pass#Wrong Answer~Mailing#Wrong Answer~Spreadsheet#Wrong answer~Holes#WrongAnswer~Game#Wrong Answer} . </p> <p>An e-mail virus moves around in {1:MULTICHOICE:Reproduce#wrong answer~Possible#wrong answer~Damage#Wrong answer~Devastating#Wrong answer~Network#Wrong answer~Software#Wrong Answer~=E-mail#Correct answer~Pass#Wrong Answer~Mailing#Wrong Answer~Spreadsheet#Wrong answer~Holes#WrongAnswer~Game#Wrong Answer}  messages, and usually replicates itself by automatically {1:MULTICHOICE:Reproduce#wrong answer~Possible#wrong answer~Damage#Wrong answer~Devastating#Wrong answer~Network#Wrong answer~Software#Wrong Answer~E-mail#Wrong answer~Pass#Wrong Answer~=Mailing#Correct Answer~Spreadsheet#Wrong answer~Holes#WrongAnswer~Game#Wrong Answer}  itself to dozens of people in the victim's e-mail address book. </p><br clear="all" /> <p>A worm is a small piece of software that uses computer networks and security {1:MULTICHOICE:Reproduce#wrong answer~Possible#wrong answer~Damage#Wrong answer~Devastating#Wrong answer~Network#Wrong answer~Software#Wrong Answer~E-mail#Wrong answer~Pass#Wrong Answer~Mailing#Wrong Answer~Spreadsheet#Wrong answer~=Holes#Correct Answer~Game#Wrong Answer} to replicate itself. A copy of the worm scans the {1:MULTICHOICE:Reproduce#wrong answer~Possible#wrong answer~Damage#Wrong answer~Devastating#Wrong answer~=Network#Correct answer~Software#Wrong Answer~E-mail#Wrong answer~Pass#Wrong Answer~Mailing#Wrong Answer~Spreadsheet#Wrong answer~Holes#Wrong Answer~Game#Wrong Answer}  for another machine that has a specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there, as well. </p> <p>A Trojan horse is simply a computer program. The program claims to do one thing (it may claim to be a {1:MULTICHOICE:Reproduce#wrong answer~Possible#wrong answer~Damage#Wrong answer~Devastating#Wrong answer~Network#Wrong answer~Software#Wrong Answer~E-mail#Wrong answer~Pass#Wrong Answer~Mailing#Wrong Answer~Spreadsheet#Wrong answer~Holes#WrongAnswer~=Game#Correct Answer} ) but instead does {1:MULTICHOICE:Reproduce#wrong answer~Possible#wrong answer~=Damage#Correct answer~Devastating#Wrong answer~Network#Wrong answer~Software#Wrong Answer~E-mail#Wrong answer~Pass#Wrong Answer~Mailing#Wrong Answer~Spreadsheet#Wrong answer~Holes#Wrong Answer~Game#Wrong Answer}  when you run it (it may erase your hard disk). Trojan horses have no way to replicate automatically. </p> <p>Computer viruses can have {1:MULTICHOICE:Reproduce#wrong answer~Possible#wrong answer~Damage#Wrong answer~=Devastating#Correct answer~Network#Wrong answer~Software#Wrong Answer~E-mail#Wrong answer~Pass#Wrong Answer~Mailing#Wrong Answer~Spreadsheet#Wrong answer~Holes#WrongAnswer~Game#Wrong Answer} effects. Experts estimate that the Mydoom worm infected approximately a quarter-million computers in a single day in January 2004. Back in March 1999, the Melissa virus was so powerful that it forced Microsoft and a number of other very large companies to completely turn off their e-mail systems until the virus could be contained. The ILOVEYOU virus in 2000 had a similarly devastating effect. </p> 0