Let's see, so, Melpomene basically gets sick for under a day, and then whatever was in there purges itself. So she doesn't really get serious viruses. However, she can experience the effects for a full 24 hours, if they start exactly at midnight.

Lamia, I don't think, is capable of any sort of human feeling whatsoever-- and that includes nausea.

Dr. Horrible just sits on his sofa and secretly wishes his mother were there, but instead tries to think comforting thoughts about bank robberies and nailing Captain Hammer to the wall.