Viruses are software programs, and they can do the same things as any other programs running on your computer. The actual effect of any particular virus depends on how it was programmed by the person who wrote the virus script. Some viruses are deliberately designed to damage files or otherwise interfere with your computer's peration, while others don't do anything but try to spread themselves around. But even the ones that just spread themselves are harmful, since they damage files and may cause other problems in the process of spreading.

Some Viruses can copy your ID's and passwords and send it to the admin or the creator of the virus.
thats when it become a big problem when it comes to the banking or online payments ID's and passwords.