This is pear rust (Gymnosporangium sabinae).
It is fairly widespread in the northern hemisphere:
It is a fungal infection and affects pear trees from junipers. Pruning the affected nearby junipers can help control its spread.
This page seems to indicate that it adheres to the epithelium of different internal organs. They write:
The sporozoites are released and parasitize epithelial cells (b, c) of the gastrointestinal tract or other tissues such as the respiratory tract.
Here is the figure from that page: