Virus? None that I know of. It's a spin-off of FH, in a way. Just more options. Some people like that better, some people like regular FH better.
Download: Same page as regular FH. Just look under "Branch Versions".
Old FH: Yup. You will be able to pick between using FH and FH+.
As of current, FH+ is having some technical issues. So you may want to wait it out a bit. I believe that more people play regular FH as it is. Just know that FH+ may cause you more headaches than regular FH. Of course, you may also have a lot more fun with it.
