Earlier computer viruses were simply a peice of code attached to a bigger more
significant program. Whenever this bigger program was ran, so was the virus.
Once the virus was ran it would load itself into RAM where it " looked " around
for other programs to infect. Now, there were two programs on the hard drive
which were infected by the virus and when either of these programs is loaded, so
was the virus.
Newer viruses are a little bit more sophisticated. Now, viruses will stay loaded
into RAM and constantly spread themselves around the hard drive, whereas before
the program containing the virus had to be executed before the virus could do any
harm. These newer viruses can even hide in the CMOS. This way the virus can stay
on the computer even after the hard drive is formatted.