Shia religious authority, Grand Ayatollah Sistani, issued Fatwa about Swine Flu:

Q: In spite of anxieties about expansion of swine flu among pilgrims in rituals of Hajj, which would lead to a big tragedy, what is your opinion about necessity of performing rituals of Hajj in this year?

Ans: it is obligatory to performing Hajj, but if its harms confirmed by wise people, after that it is not obligatory to performing it.