At a former workplace there were pigeons nesting in the wall behind me. We could hear them scratching about initially then a few of us started getting bitten by something.
After getting some bites on my foot I ended up with a bad case of cellulitis which took a few doses of very strong antibiotics and lying with my foot above my head for 48 hours to clear up. I reckon they were some kind of pigeon mites/ticks.
Thankfully, after that more of a effort seemed to be made to stop them getting in the space. The few times they came in to sort it though the smell was absolutely horrendous, they pulled a dead bird or two out and this stagnant water and it was disgusting.