When a
virus get activated as a result pf executing a virus infected program, the
virus tries to copy itself into uninfected programs in the same computer or in
other computers connected to it over a network. And repeatedly the newly
infected programs try to infect yet more programs. Removable media, such as
floppy diskettes and USB sticks has remained in the position as a major source
of virus carriers form the past.
The third major source of viruses
is email attachments. According to ICSA, infection that spread through e-mail
attachments. The source of macro viruses, increased from 2 percent in year 1998
to over 60 percent in year 2000.