A incomplete program , which is intentionally made for infecting other programs is known as computer virus.
(a) Append: In this method viral code appends itself to the unaffected program.
(b) Replace: In this method viral code replaces original executable program
completely or partially.
(c) Insert: In this method the viral code is inserted in the body of an executable code
to carry out some funny actions.
(d) Delete: In this case viral code deletes some codes from executable program.