Repost: The Toughest Logic Puzzle

This is a repost from two years ago. That post had become inundated with splogger comments, so I'm reposting it. I’ve taken the liberties (or perhaps you view it as an injustice if you are the original author) to rewrite a puzzle I once heard. I can find no reference to it online, so here goes.
Tale of the Diseased Monks
One Sunday evening after working in the fields, the secluded monks of the Kaetorsian order gathered for evening prayers. After the usual somber songs and pious prayers, the high priest said, “I have a grave announcement. It appears a horrible disease has fallen on our community this fine spring day. I know this because the disease results in a purple spot on your forehead, and I can see that some of you have this. From what I know of this most evil disease - you will remain unharmed for 14 days. After which, the disease will spread to others, and you will experience a most painful passing that may last months. If we are not careful, this disease will completely destroy our peaceful monastery. Therefore, I ask that those of you who have this spot to please remove yourself from our community immediately. I pray (for my sake!) that this matter will be resolved before these two weeks are over. Despite the fact that all of you have taken a vow of silence, and a vow of humility, and thus will not be able to inform one another of the forehead spot, and even though we lack mirrors and the lake is choppy and you are unable to see for yourself whether you have this spot, you are all trained highly in the ways of logic and will be able to deduce on your own whether or not you have become infected. In this way, we will carry on as we always have: working solitarily all morning and congregating here every evening to share in this holy life. Some of you would prefer that I simply point out those of you who are diseased and while I have not taken the vow of silence that you have taken, my vow of humility prevents me from calling attention to your dysfunctions. Good night”
The next few days passed as they always have. The monks that had been infected seemed as good natured as the others, and no one treated one another any differently. However, after more than a week, as the two week deadline approached, an air of nervousness crept in. The second Saturday after the high priest’s announcement was particularly tense. The next day marked the two week deadline before the disease was to spread again, and the diseased monks were still working and praying along side the healthy ones. After the congregation disbanded from the Saturday evening prayers, the monks returned to their private quarters. On Sunday, two weeks after the high priest announcement, all of the diseased monks were gone. Through their highly tuned logic skills, they were able to determine that they had been affected and sacrificed themselves for the good of the monastery.

How many monks were infected (the actual number, please)? And how did they determine it?

My only hint is the following. What would you notice if you were the only one infected?