I was aching for a dark book and this definitely delivered, especially at the end as the situation quickly deteriorated. The concept of the Tox is fascinating and I like how slowly the truth about its nature is revealed (both how it affects everyone exposed to it and why it is on Raxter in the first place). 

However, there's several things I find unbelievable about the story,

from how the doctors didn't seem to have figured out a parasite was at work to Welch not trying to escape the island earlier or try to contact the outside world. The headmistress was also not brought up enough for the role she played at the end.