The faraway monastery

Since I love riddles, I’ll try to post the one I enjoy from time to time. Good luck for solving!

The story takes place in a faraway monastery. This monastery is located in a high mountain, and rules are really drastic: The monks cannot communicate each one with each other, in any way. (chat, written, signs, sounds… all of this is prohibited) And the second rule is the absence of any mirror. The monks are supposed to reach a very high spiritual level, and thus, are not allowed to think about their physical body anymore.

All the monastery has been built on those rules. There are not many big places, so it makes meeting hardly possible, there are no mirror, no reflecting objects, and the water in the well is low enough to prevent any water mirroring.

The only exception Is the diner time. Everyday, all the monks meets up arround a round table, and have diner there. While they still cannot communicate during this time, at least they can see each other.

But once, during the diner, the priest stood up and said:

My dear brothers. I come with a sad news today. I am sorry to break my silence vow, but I feel I had no other choice.

Some of us got a deadly virus. There is no antidote, and the infected people will undoubtfully die. As soon as we noticed the presence of this virus, we added a potion in your wine, so we could stop the epidemia.

However, this potion cannot cure the affected people. The affected brothers won’t feel anything. The only visible sign of this decease is a big red spot on the forehead.

So I’m asking all the sick brothers to leave the monastery, and try to go to the nearest hospital.

However, since we already broke our rules tonight with this speech, no more exception will be tolerated. From now, you shall keep on your vows.

So… How will the sick monks notice they are actually sick?