User blog comment:FairyRaven/Fairy Tail Chapter 428: When We Take Different Paths, Review and Commentary/@comment-76.121.234.236-20150415065247/@comment-25111584-20150417060124

Just like one can't apply normal biological knowledge to a Slayer's body, one can't apply it to Juvia's body. She has stated a number of times that her body is made of water. Shown proof too. And she can turn the water boiling hot. Her getting sick due to water when her body is water doesn't make any sense, especially after her statement of being strongest when in the rain (or an aquatic environment).

This has no part in the discussion, but ... all Slayers are almost immune to their elements. Too specific? Even as an Ice-Make Mage, Gray, is immune to the cold. Juvia not being immune to her own element, an element of which her own body is composed, is kinda like trolling Juvia, something which happens 10x more than any other 'serious' character in the series.

XD If you're talking about her umbrella ... look at it closely. It's one of those personal parasols, mainly meant to be used as a sunshade. It'll help you a bit in a drizzle or a short distance in the rain, but it's normally useless as an umbrella. Though it'll obviously be of some use as it is a canopy above your head. Basically, consider it as a forlorn hope that the sun will shine on her again someday rather than her actually needing to use it. ;)