Can someone explain this to me?

I wouldn't recommend relying on something as tenuous as a second hard drive to help you prevent damage from a virus. Any Virus that you get is likely to start damaging your system drive regardless of where it is installed. The purpose of havng more than one drive has more to do with what type of drives you use than it does with virus protection. If you want to protect yourself from a virus infection, get a good virus scanner program and install it, be very careful about the source of programs you install on your computer and use good judgement when visiting websites that could be engaging in questionable activity. A good virus program will provide warnings about suspicious websites and programs.

Now to explain the real purpose of having more than one drive. Basically, to get better performance, you want to get an SSD with a capacity between 60GB and 256GB(120GB and above is preferred) to use for your Operating System and most used programs and games. SSD's are much faster than mechanical hard drives, but they are also much more expensive, so you want the programs that will benefit the most from high speed data transfers to be installed there. The HDD should be AT LEAST 2-4 times larger than the SSD. It is used to store less used programs, programs that don't benefit from the higher speed and all of your data (Pics, Videos, Documents and other data files).

I hope that helps answer your questions.