I got a lot of information from this lecture. The Avian flu was harmless to waterfowl, but when humans started raising chickens in <factory farms> where the chickens are confined too close to one another, the flu virus spread to infect their lungs. The confined chickens allow the virus to spread rapidly through the flock, giving the virus a massive opportunity to mutate. Then through mammals (pigs), the flu infected humans. The reason why mankind has more diseases is because of factory farming — chickens, swine, livestock.