PC viruses and their characteristics

Logic bombs — Programmed threats that lie dormant for an extended period of time until they are triggered; at this point, they perform a function that is not the intended function of the program in which they are contained. They are often triggered by a PC’s internal clock.

Viruses — "Programs" that modify other programs on a computer, inserting copies of themselves. Viruses are not distinct programs – they cannot run on their own, and need to have some host program, of which they are a part, executed to activate them.

Worms — Programs that propagate from computer to computer on a network, without necessarily modifying other programs on the target machines. Worms can run independently and travel from machine to machine across network connections; worms may have portions of themselves running on many different machines. Worms do not change other programs, although they may carry another code that does (for example, a true virus).

Trojan horses — Programs that appear to have one function but actually perform another function. Trojan horses are named after the Trojan horse of myth. Analogous to their namesake, modern-day Trojan horses resemble a program that the user wishes to run – a game, a spreadsheet, or an editor. While the program appears to be doing what the user wants, it is also doing something else unrelated to its advertised purpose, and without the user’s knowledge.

Bacteria, or rabbit programs — These make copies of themselves to overwhelm a computer system’s resources. Bacteria do not explicitly damage any files. Their sole purpose is to replicate themselves. A typical bacteria program may do nothing more than execute two copies of itself simultaneously on multiprogramming systems, or perhaps create two new files, each of which is a copy of the original source file of the bacteria program. Both of these programs then may copy themselves twice, and so on. Bacteria reproduce exponentially, eventually taking up all the processor capacity, memory, or disk space, denying the user access to those resources.

Dropper — This is a program that is not a virus, nor is it infected with a virus, but when run, it installs a virus into memory, on to the disk, or into a file. Droppers have been written sometimes as a convenient carrier for a virus, and sometimes as an act of sabotage. Some anti-virus programs try to detect droppers.

(Source: College of Engineering and CS, Wright State University, Dayton, Ohio - < ahref="http://www.cs.wright.edu">www.cs.wright.edu)