Fair question, jhardy. Concentrating animals of the same species can accelerate disease spread if/when it enters that local population. Same is true for domestic animals. Just ask a poultry farmer how quickly things can spiral from a few sick birds to loss of an entire chicken house. Also true for viral infections in humans. We know the stories personally. A student contagious with the flu goes to school. Within days, flu virus is sweeping through the school. Concentration accelerated transmission from one infected kid to a large number of students and faculty, who then spread it to others at home. You may recall schools last winter that closed for several days. Why? To break the cycle by reducing concentration of people. But, if that same sick kid stays home with minimal contact with others, then it slows or even prevents the spread.