From what I heard in an NPR reenactment or interview last year, the VDs on the L&C Expedition did play a big part in it.
Yeah. I haven't done anything resembling research or even reading up, but it seems like A. VD was yet another disease "gift" from the white folks that did terrible things to the indigenous population and B. (this is where I'm on shakier ground) some people's take is that Native Americans had different social morays when it came to physical intimacy, to the point where a more prudish culture would think it promiscuity. Or at least there was certain types of "sharing" that was acceptable.
I'm on total shaky ground when it comes to B, but I think, in general, VD issues could probably cause all types of logistical issues for an expedition.
The Lewis and Clark thing reminds me of this episode of "This American Life,"
http://thislife.org/Radio_Episode.aspx?episode=329
in the third act of which Sarah Vowell talks about frontier cartographer Charles Preuss, who charted the Western Territories of the US, but, per his diary, hated every second of it. It's a funny idea.
TAL also has an episode, "The Book That Changed Your Life,"
http://thislife.org/Radio_Episode.aspx?episode=137
which includes an act about a man who wants to collect every book ever written about the Lewis and Clark expedition.