Classification of viruses according to the way they act or methods of propagation:
worms. – They are programs that reproduce themselves and do not require a host program or file. Because it replicates, this type of intruder can decrease the capacity of main memory or a disk; can slow down data processing. Some experts do not classify it as a virus because it does not destroy files and does not require a host to spread. They affect systems connected in networks.
Time bombs. – They are viruses programmed to go into action at a predetermined time, a particular time or date. They hide in the computer’s memory or on disks, in files with executable programs with the .EXE and .COM extensions. When the time is right, they “explode,” displaying a message or doing the damage they were programmed to do. They are activated when the program that contains them is executed.
Trojan horses. – They are viruses that need to be copied and installed manually on the computer that will be infected, destroying the information on the disk. They enter the system under a completely different guise from their final target, like the original Trojan Horse. After some time they become active and show their true intentions. For example, a “disguised” program may be presenting the user with some nice graphics on the screen and on the other hand it is destroying the contents of some file or it is reformatting the hard drive.
Concealment virus or Stealth (Ghosts). – They are viruses that try to camouflage themselves to avoid being detected and eliminated. Among the mechanisms that these viruses use to achieve their goal are: hiding certain files and directories, hiding the virus itself in memory, and hiding the actual size in bytes or kilobytes of the infected files.
polymorphic viruses. – It changes itself each time it propagates, so when trying to identify it by searching for a sequence of bytes that characterize it, it is not found. It changes its content randomly and makes it difficult to identify.
Retro Viruses. – These are viruses that directly attack the antivirus that is on the computer. Generally, what it does is that it looks for the virus definition tables of the antivirus and destroys them.
Virus Backdoors. – They are also known as hidden remote administration tools. These are programs that allow remote control of the infected computer. They are generally distributed as Trojans.
False Virus or Hoax. – These are the e-mail chains that generally announce the threat of some “very dangerous” virus (which never exists, of course) and that, out of fear, or with the intention of preventing others, are sent and re-sent incessantly . This produces a senseless state of panic and generates annoying traffic of unnecessary information.