Cooney is great at suspense, but really there was no good way for this book to end. After going into detail of the rather awful nature of small pox, the main character who may/may not have been exposed can have the following happen:

1. He can not come down with the disease which in a strange way would be disappointing after all the hype.
2. He can come down with the disease, or kill himself to save the world, which in a strange way would be disappointing because he is, after all, the main character.
3. He can be part of some other strange twist which would be disappointing because it wouldn't be realistic.

So there you have it - no spoilers, just prepare for a disappointing ending. On the other hand, the students in the book are working on a project quite similar to the one I'm inflicting on my students which made it fun for me to "compare notes".