Why do viruses mutate?
Bird flu vs. swine flu Almost all human diseases originate from other animals and then adapt to human hosts, says University of Guelph agriculture professor David Waltner-Toews, author of The Chickens Fight Back: Pandemic Panics and Deadly Diseases that Jump from Animals to People. With influenza, different strains of the virus affect people, birds and pigs. Naturally, human influenza is the easiest for humans to get and to transmit to other humans. Humans can also get bird influenza and pig influenza, but it typically requires very close contact with the animals, Waltner-Toews said . Bird or avian influenza is particularly hard to get, but humans who do get it experience very severe symptoms. That is why people were worried about a deadly pandemic if avian influenza were to mutate in such a way as to become easily transmissible between humans. Swine influenza is somewhat easier to transmit to humans but its symptoms tend to be milder than bird influenza. Most flus contracted by humans
