yeah, perhaps a simple example would help.
let's take a n exceptionally tough orc, say, bod 8. while this orc is really healthy, he doesn't really have access to a lot of tech, most of which won't work anyways (penetration -6) and we're going to ignore the possibility of using O-cells entirely because i don't want to get rabies from the rabid rules lawyers.
so, mr Orky McHealthy gets exposed to HMHVV and now has to worry about turning into a ghoul.
so, first thing first, we start off with the basics: power 8, penetration -6, the test to resist the disease is body + (stuff that boosts this dice pool which we aren't using). a minimum of 10 tests must be made, each test is 1 day apart, and once 10 tests have been failed (ever, in your entire lifetime) that's it, you're a ghoul.
so, 8 dice, needing 8 hits. for the sake of argument, we'll give our ork 10 edge (i don't think this is even possible, but for the sake of argument, it could be 12 body and 6 edge, refresh daily, for all i care. the point is how absurdly impossible this is to resist). apart from the fact that he just got infected with a nigh-unstoppable disease, Orky McHealthy might almost be described as Mr Lucky.
so anyways, he's gonna blow edge on his first 10 resistance rolls. 18 exploding dice, whee!
for convenience, here are the die rolls he'll be using:
http://invisiblecastle.com/roller/view/2814628/ (will be gone if you don't check it before it disappears)
so, first day, he did pretty good. 7 hits! that's impressive. but, uhh.... too bad he needed 8. that's 0.1 essence lost, and the disease now gained 1 point of power (and is now power 9).
second day, still did pretty good. 6 hits. unfortunately, today's threshold is 9. too bad, he failed again, and the disease is now power 11. he has now lost 0.2 essence total.
day three, wow! 10 hits! ... oh, and uhhh... not enough. the disease had accumulated to power 11, so tomorrow it will be power 9. and he's lost 0.3 essence.
day four, and this is not his lucky day. 4 hits, not even close. he's lost another chunk of essence (0.4 total), and tomorrow the disease will be power 13.
day five, and he gets 6 hits. unfortunately, he needed 13, a difference of 7 hits, meaning tomorrow the disease will be power 15. and of course, that's 0.5 essence lost now.
day 6, he gets 7 hits. pretty good. the disease is now power 16, though, and 0.6 essence has been lost.
day 7, he gets 6 hits again. the disease is now power 18, and 0.7 essence has been lost.
day 8, he gets 11 hits. not bad. nowhere near good enough either. but at least the disease is now "only" power 15 again, and he's got 0.8 essence lost...
day 9, he gets 1 hit. i'd say this was the beginning of the end, but honestly the beginning of the end was the second he didn't shoot the ghoul with a sniper rifle while wearing a biohazard suit from orbit. so uhh... power 22, 0.9 essence lost.
day 10, unsurprisingly not 22 hits. it is in fact, 8 hits. keeping the power at 22. and he's lost 1 point of essence, and is now a ghoul. on the plus side, the disease has run it's course, having done as much as it will ever do. he can now safely interact with ghouls, provided neither he nor they lose their mind. which is good, because if you play with how the rules say instead of how they've been stated as being intended (that is, if you don't know the disease is supposed to be injection not contact vector) pretty soon everyone in the world is going to be a ghoul, with very few exceptions
and this is with someone that got to throw *18 exploding dice* every single time.
but hey, maybe he just didn't roll very well. let's check how effective that is in resisting the disease over, say, 15 days. i'll do 8 trials, shall i? this is hardly *proof*, but it will likely show us a very simple trend:
http://invisiblecastle.com/roller/view/2814634/ (ghoul in 13 days)
http://invisiblecastle.com/roller/view/2814638/ (0.6 essence lost in 10 days, disease resisted... the lucky dog)
http://invisiblecastle.com/roller/view/2814642/ (ghoul in 11 days)
http://invisiblecastle.com/roller/view/2814646/ (ghoul in 13 days)
http://invisiblecastle.com/roller/view/2814648/ (ghoul in 12 days)
http://invisiblecastle.com/roller/view/2814653/ (ghoul in 10 days - forgot to relabel, this was still listed as resist 5)
http://invisiblecastle.com/roller/view/2814655/ (0.9 essence lost in 15 days - disease rages on at power 12, he's out of edge so he would need 12 hits on 8 non-exploding dice. yup, he's a ghoul all right. actually, he got an extra 5 days of edge already...)
http://invisiblecastle.com/roller/view/2814662/ (ghoul in 10 days)
and this is with *18* exploding dice... on a superlative example of metahumanity, no less (even trolls won't often have much more bod than 8, and as i said... 10 edge is essentially unfeasible)
so yeah, the disease is *extremely* difficult to resist. an extremely healthy individual with impossibly high edge has to roll exceptionally well just to be able to resist it succesfully.