I thought that bee propylis protects them from all the dangers of disease that come from living in large colonies. What's going wrong?

Re the original question: the answer used to be my sitting room. In a cottage I used to live in, many mortar bees laid eggs inside the chimney. The newly-hatched bees used to get confused between the light at the top of the chimney and the light in my sitting room, and were as likely to find themselves in my house as outdoors. We often found dead bees by the window, and I started wearing slippers after the second time I got stung on the foot.

Then there was a whole dead swarm in the sitting room once when the house had been empty for a couple of weeks. Those weren't mortar bees - they were ordinary honey bees, I think. The pest control man assured us we didn't have a hive in the chimney, but somehow the swarm had found it's way down the chimney. Bizarre. He also showed us how to gently pick up and release the newborn and confused mortar bees outside, instead of letting them die. Awwwww...

I always found it very sad that bees died in my house. I like bees. I think they're lucky.