Basically, all they do is change the ID of the partition on disk. You can change it to say "Hidden FAT32", which effectively hides the partition. The only way to "see" the partition is to use an OS/program that doesn't recognize that the partition ID is "hiding" the partition-- Linux, for instance, does this, I believe that you can see the partitions in 2000/XP, too.
But, the partition should be safe from viruses. The image on disk is a file, which is kind of hard to corrupt by a virus. Doing so would require the virus writer to have in-depth knowledge of the image file format.
Basically, all they do is change the ID of the partition on disk. You can change it to say "Hidden FAT32"...
Yup. When you first create/format a partition, it is marked with a two-digit Hexidecimal ID which identifies attributes of the partition such as the filesystem type (FAT, FAT32, ext2, etc.), whether the partition is Primary or Extended, and other things. The "hidden" attribute is available for most filesystem types, and can be changed with a number of tools, including the Linux version of fdisk.