exactly. before the infection test can even happen, you must first be drained to 0 essence. therefore, you're losing at least 6 points of essence, which equates to at least 6 points of magic, which equates to at least 12 points of immunity to disease. of course, if immortal elves have more than 6 points of essence, this is just going to be even worse for the immortal elf, but SR4 seems to have given everyone equal essence (except for stuff that has variable essence).
It seems to me like IE's would have more than 6 Essence, unfortunately for them. Dragons all have 7 or more essence, and iirc its hinted in the SR and ED canons that the IE's are dragonkin - which would probably raise the IE's essence by virtue of being descended from dragons. Of course, never having played ED, my understanding of the term 'dragonkin' may be incorrect.