Biological vs. Computer Viruses

To get a better idea of how a virus can attack a computer, think of the computer as a walking, talking, living person and the computer virus as a biological virus. To aid in this analogy, we've included a table written by Ralf Burger and Rune Skardhamar as a comparison between Biological Viruses and Computer Viruses. Please note, this table refers to an IBM-PC Compatible computer running the MS-DOS operating system:

Biological Viruses Computer Viruses
Attacks specific body cells Attacks specific programs (all *.COM, all *.EXE, etc.) [in a MS-DOS environment]
Modify the genetic information of a cell other than originally intended Manipulate the program: [so that] It [the program] performs tasks [not originally intended]
New viruses grow in the infected cell itself The infected program produces virus programs
An infected cell is not infected more than once by the same cell A program is infected only once by most viruses
An infected organism may not exhibit symptoms for a long time The infected program can work without error for a long time
Not all cells with which the virus comes in contact are infected Programs can be made immune against certain viruses
Viruses can mutate... [so that they] cannot be clearly told apart Virus programs can modify themselves and possibly escape detection this way

While this table give you an idea of how a computer virus is similar to a biological virus, it is not entirely correct.

In the biological systems, a virus can infect a cell more than once. In fact, a cell may be infected by many viruses at the same time. This creates the possibility of viral interactions leading to mutations of the viruses. A similar effect may happen in a virus infested computer system. Several different computer viruses may infect the same program at the same time. While in biological viruses this may result in new mutations, in computer viruses this usually results in a non-functional virus and a corrupted file.

Rune Skardhamar, author of VIRUS Detection and Elimination, has created a slightly more up to date comparison chart (included below).

Biological Viruses Computer Viruses
Biological viruses are parasitic. They need a cell in which to live and reproduce. Computer viruses are parasitic. They need another program to exist and in which to reproduce.
Once a cell has been infected, the infected cell is forced to make new copies of the virus. Once a program has been infected, it is forced to make new copies of the virus.
Biological viruses rarely, if ever, infect the same cell twice. Computer viruses seldom infect the same program/disk twice.
Specific biological viruses target specific cell types. Specific computer viruses target specific program types (EXE, COM, SYS, etc.).
An infected cell can go on living for a long time without any obvious sign of infection. An infected program need not show any obvious sign of the infection.
After an incubating time of varied length, a biological virus often releases some kind of payload, which can prove fatal to the whole living system in which the infected cell lives After an incubating time of varied length, a computer virus often releases some kind of payload, which can prove fatal to the whole system in which the virus-infected program lies.
The DNA of a typical small virus, such as the polio virus, contains information that if reproduced on a computer would add up to some 5,000 bytes. The smallest virus discovered to date is equal to about only 200 bytes. A typical computer virus has a size of some 1,000-3,000 bytes. However, if the DOS interrupt subroutine (called by the int instruction) is added to that, the full size can easily be multiplied by two (And that's not even counting the microcode actually being executed by the CPU.)