The Frodo virus is one of the most advanced "stealth" viruses, and is quite successful in hiding from detection. If it is active in memory and you look at the directory, the virus will show you the original length of any infected program. In addition, it will intercept most attempts to read infected files, so only the original, non-infected file will be seen.
If CHKDSK is run while the virus is active, it will often detect an inconsistency in the number of disk blocks allocated to infected files, and their apparent length.
Frodo activates on Sept. 22. when it may attempt to place a Trojan on boot sectors. This Trojan will display the message "FRODO LIVES" in large letters on the screen, surrounded by a moving pattern. The code to write the Trojan to the disk seems to be garbled in all known versions of the virus and will probably "hang" the computer.