Like all fecal matter (human and animal) pigeon droppings can contain bacteria that are harmful if ingested, so it is best to keep pigeon droppings away from food that is about to be eaten and from kitchen work surfaces.
The fungus histoplasma capsulatum, if inhaled, can cause histoplasmosis in humans. This isn't likely to happen in the UK because the fungus has not been found in our soil. Pigeons don't carry or spread histoplasma, it is not a disease that can be caught from a bird but under the right conditions (temperate climate, damp acidic soil with high organic content) all bird droppings can produce an environment in which this fungus thrives if it is there in the first place.
Cryptococcus neoformans is another fungus that is found in the dried droppings of birds, including pigeons. When dried bird droppings are stirred up, this can make dust containing Cryptococcus Neoformans go into the air. If inhaled this doesn't normally affect healthy humans but it can cause disease in people with compromised immune systems, such as those with HIV/AIDS, people who have had organ transplants and those who are being treated for cancer.
Pigeon droppings, which are admittedly unsightly against the clean wall of a building, are used as an excuse to kill them.
Pigeon droppings used to be used as a dowry because they break down into a wonderful fertiliser for green vegetables in a very short time.
Recently I have heard pigeon droppings described as "toxic" (by a pest controller, of course). They are nothing of the sort! This is what I do with pigeon droppings:
I use a scraper to collect the them every day from my rescue birds' aviary .
I put the fresh droppings in my garden composter and wait for it to break down, which doesn't take long.
Then it is used by my neighbour who grows green vegetables on his allotment . In return he provides me with some of his harvest.
I can recommend this as a simple, green, money saving solution!