The dBase virus is very rare, but rather curious. It is clearly intended to garble dBase files, or rather any file with a name that ends in .DBF.
If the virus is active in memory when a program writes to a .DBF file, it will garble all the outgoing data. However, when the data is read back later, the virus will correct the garbled data.
There is just one problem. If the virus is detected and removed, the data will be useless because the virus will not be present to "de-garble" it when it is read back.
There is a more harmful side to this virus. If an attempt is made to write to a .DBF file that is more that three months old, the virus will try to destroy the FAT and root directory on drives D:, E: .... Z: There is a bug in the code, however, so the destruction will be rather unpredictable.