As it goes, the forums probably will not give you a virus. Not unless you're clicking on the ads scattered throughout. However, constantly downloading files from Mediafire WILL give you viruses, as long as you're not careful about what you're downloading and the source of finding the link.
However, uploading files to Mediafire will NOT give you a virus, in most cases, as it is coming from your computer to the Mediafire server. And as long as your computer is virus-free, those who download your map will also remain virus-free.
Forums: Safe was we know it. Since there hasn't been a vast virus outbreak, zombie style.
Mediafire: Safe for uploads, careful on downloads.