Original: Because these symptoms are not specific to swine flu, a differential diagnosis of probable swine flu requires not only symptoms but also a high likelihood of swine flu due to the person's recent history.
Simplified: The current strain of swine flu can adapt to humans and spread more efficiently than previously known swine H1N1 strains. Moreover, co-infection of H1N1 swine flu and Oseltamivir resistant H1N1 season flu can lead to acquisition of the H274Y mutation by the swine flu via recombination or reassortment.