The issue is not so much with the pigeons as it is with what they leave behind – bird mites, debris, nesting material and worst of all, a toxic mixture of urine and feces. When all is said and done a flock of just 100 pigeons can deposit up to 4,800 pounds of waste, annually.
Pigeon droppings also contain a high concentration of uric acid which is highly corrosive and will melt through just about anything. Under the right set of conditions it will even corrode steel and concrete.
WHO recently published the guide which explains 60+ different diseases associated with pest birds, most prominently pigeons. The health risks should not be underestimated.