No, it's not a virus. If you download it from the official link, we guarantee you that we do not add anything bad.
All the code is open source and free to inspect, also the build procedures used to create the executable are included in the respository and we guarantee that these are the ones being used for creating the nightly builds or releases, without secret additions.
Some virus scanners tend to have what we call "false positives": they mark a program as malware when it is not. The reason for this is probably that they make their filter too wide, so that more things get caught up then there should be.
We are aware that Norton does not like us. Have a look here:
http://www.makehuman.org/doc/faq/norton ... gives.htmlI guess we didn't pay the people at Norton enough money

Sometimes I think that these virus scanner creators are just as bad as the people that create malware and viruses (that especially goes for Norton).