Common viruses have many different properties:
Viruses can be polymorphic - Some viruses can modify thier own code. This is done
so the virus can hide itself in many different ways. Most virus programs look
for a specific program method in a virus, if the virus can alter it's code it is
easier for it to hide from virus scanners.
Viruses can be memory resident or non-memory resident - Viruses can infect a
computers RAM where they run in the background seeking and altering files to
prepare for thier attack. Other viruses can just infect one file everytime it is
ran. This allows the virus to strategically place itslef around the hard drive,
going undetected until it's attack, almost like the strategy of the " Aliens " in
the movie " Independence Day."
Viruses can carry other viruses - Each virus is programed by a different person,
therefore viruses cannot destinguish between important files on the hard drive
from other viruses on the hard drive. When one virus attacks another virus, it
becomes one big virus, therefore, if it is transferred to another computer it will
hit it twice as hard.
Some viruses can hide thier effects - A virus can change the size of files it
has attacked and likewise, it can hide these changes so they hopefully go
undetected.