The flu can jump from animals to humans. A new strain of flu has emerged four times in the past 100 years. The one that emerged in 1918 killed 50 million people, which is why scientists watch these new flu strains so carefully.
Last fall, over 150 harbor seal pups washed up on the shores of New England beaches. Researchers discovered that they were killed by a new strain of flu, that evolved from bird flu. It had obviously gained the ability to spread from seal to seal.
In the July 31st edition of the New York Times, Carl Zimmer quotes biologist Katie Pugliares as saying, "Surfers were surfing into seals floating in the water."