https://www.backyardchickens.com/articles/red-mites-can-transmit-diseases-tips-for-prevention-and-monitoring.76285/
I originally wrote this article after reading about a research on transmitting diseases. This part is translated from a Dutch article. The other info is from gathering knowledges...
Sorry to burst into this Welshmen. Perris gave excellent answers so far. But
maybe I can add a few ideas and a response to your question about gheir immune system.
Healthy supplements for chickens I give my chickens now and then: garlic, apple cider vinegar, oregano. But there are more...